Class-10th PHYSICS

STUDY NOTES & WORKSHEET

LIGHT

LIGHT : REFLECTION and REFRACTION

1. NATURE OF LIGHT

There are two theories about the nature of light: wave theory of light and particle theory of light. According to wave theory: Light consists of electromagnetic waves which do not require a material medium (like solid, liquid or gas) for their propagation. The speed of light waves is very high (being about 3×108 metres per second in vacuum). According to particle theory : Light is composed of particles which travel in a straight line at very high speed. The elementary particle that defines light is the ‘photon’. Light has a dual nature (double nature): light exhibits the properties of both waves and particles (depending on the situation it is in).

2. REFLECTION OF LIGHT

The process of sending back the light rays which fall on the surface of an object, is called reflection of light. The objects having polished, shining surfaces reflect more light than objects having unpolished, dull surfaces. Silver metal is one of the best reflectors of light. A ray of light is the straight line along which light travels. A ‘bundle of light rays’ is called a ‘beam of light’.

2.1 Reflection Of Light From Plane Surfaces: Plane Mirror

The incident ray, reflected ray and the normal to the reflecting

surface lie in the same plane.

The ray of light which falls on the mirror surface is called the incident ray. The ray of light which is sent back by the mirror is called the reflected ray. The ‘normal’ is a line at right angle to the mirror surface at the point of

incidence.

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The angle of incidence is the angle made by the incident ray with the normal at the point of incidence. The angle of reflection is the angle made by the reflected ray with the

normal at the point of incidence.

2.2 Laws of Reflection of Light

(1) First Law of Reflection. According to the first law of reflection of light: The incident ray, the reflected ray, and the normal (at the point of incidence), all lie in the same plane.

(2) Second Law of Reflection. According to the second law of reflection of light: The angle of reflection is always equal to the angle of incidence.

i r A ray of light which is incident normally (or perpendicularly) on a mirror, is reflected back along the same path (because the angle of incidence as well as the angle of reflection for such a ray of light are zero).

2.3 Regular Reflection and Diffuse Reflection of Light

In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction. Regular reflection of light occurs from smooth surfaces like that of a plane mirror (or highly polished metal surfaces). A plane mirror produces regular reflection of light. Images are formed by regular reflection of light. In diffuse reflection, a parallel beam of incident light is reflected in different directions. The diffuse reflection is also known as irregular reflection or scattering. The diffuse reflection of light takes place from rough surfaces like that of paper, cardboard, chalk, table, chair, walls and unpolished metal objects. A sheet of paper produces diffuse reflection of light. No image is formed in diffuse reflection of light.

Parallel rays incident on Rays reflected from Regular reflection an irregular surface irregular surface Objects and Images

Anything which gives out light rays (either its own or reflected by it) is called an object.

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Image is an optical appearance produced when light rays coming from an object are reflected from a mirror (or refracted through a lens). An image is formed when the light rays coming from an object meet (or appear to meet) at a point, after reflection from a mirror (or refraction through a lens).

Real Images and Virtual Images The image which can be obtained on a screen is called a real image. The image which cannot be obtained on a screen is called a virtual image.

A virtual image can be seen only by looking into a mirror (or a lens).

Formation of Image in a Plane Mirror Image of an Extended Object (or Finite Object) in a Plane Mirror 2.4 Lateral Inversion

When an object is placed in front of a plane mirror, then the right side of object appears to become the left side of image; and the left side of object appears to become the right side of image. This change of sides of an ‘object’ and its ‘mirror image’ is called lateral inversion. The phenomenon of lateral inversion is due to the reflection of light.

Left hand appears on the right side in the image

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Characteristics of an image formed by a plane mirror.

(a) The image formed in a plane mirror is virtual. It cannot be received on a screen.

(b) The image formed in a plane mirror is erect. It is the same side up as the object.

(c) The image in a plane mirror is of the same size as the object.

(d) The image formed by a plane mirror is at the same distance behind the mirror as the object is in front of the mirror.

(e) The image formed in a plane mirror is laterally inverted (or sideways reversed).

Uses of Plane Mirrors (i) Plane mirrors are used to see ourselves. The mirrors on our dressing

table and in bathrooms are plane mirrors. (ii) Plane mirrors are fixed on the inside walls of certain shops (like

jewellery shops) to make them look bigger. (iii) Plane mirrors are fitted at blind turns of some busy roads so that drivers

can see the vehicles coming from the other side and prevent accidents. (iv) Plane mirrors are used in making periscopes.

3. SPHERICAL MIRRORS: (REFLECTION OF LIGHT FROM CURVED SURFACES)

A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass.

Spherical mirrors: the shaded side is non-reflecting

(i) A concave mirror is that spherical mirror in which the reflection of light takes place at the concave surface (or bent-in surface).

(ii) A convex mirror is that spherical mirror in which the reflection of light takes place at the convex surface (or bulging-out surface).

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Important definitions related to Spherical Mirrors

The centre of curvature of a spherical mirror is the centre of the hollow sphere of glass of which the mirror is a part. The centre of curvature of a concave mirror is in front of it but the centre of curvature of a convex mirror is behind it.

The radius of curvature of a spherical mirror is the radius of the hollow sphere of glass of which the mirror is a part.

The centre of a spherical mirror is called its pole. The straight line passing through the centre of curvature and pole of a

spherical mirror is called its principal axis. That portion of a mirror from which the reflection of light actually takes

place is called aperture of the mirror. The principal focus of a concave mirror is a point on its principal axis to

which all the light rays which are parallel and close to the axis, converge after reflection from the concave mirror. A concave mirror has a real focus. The focus of a concave mirror is in front of the mirror.

The focal length of a concave mirror is the distance between its pole and principal focus.

A plane mirror neither converges parallel rays of light nor diverges them. The focal length of a plane mirror can be considered to be ‘infinite’ or ‘infinity’ (which means very, very great or limitless).

Relation between Radius of Curvature and Focal Length of a Spherical Mirror

The focal length of a spherical mirror (a concave mirror or a convex mirror) is equal to half of its radius of curvature. If f is the focal length of a spherical

mirror and R is its radius of curvature, then: 2Rf

3.1 Rules for obtaining Images formed by Spherical Mirrors:

The image is formed at that point where at least two reflected rays intersect (or appear to intersect).

Rule 1. A ray parallel to the principal axis, after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the

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principal focus in case of a convex mirror. This is illustrated in the following figures:

Rule 2. A ray passing through the principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis. This is illustrated in the following figures:

Rule 3. A ray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror, after reflection, is reflected back along the same path. This is illustrated in the following figures. The light rays comeback along the same path because the incident rays fall on the mirror along the normal to the reflecting surface.

Rule 4. A ray incident obliquely to the principal axis, towards a point P (pole of the mirror), on the concave mirror or a convex mirror, is reflected obliquely. The incident and reflected rays follow the laws of reflection at the point of incidence (point P), making equal angles with the principal axis.

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3.2 Formation of different types of Images by a Concave Mirror:

The type of image formed by a concave mirror depends on the position of object in front of the mirror. The object can be placed at different positions (or different distances) from a concave mirror to get different types of images. For example, the object can be placed:

(a) between the pole (P) and focus (F), (b) at the focus (F), (c) between focus (F) and centre of curvature (C), (d) at the centre of curvature (C), (e) beyond the centre of curvature (C), and (f) at far-off distance called infinity.

(a) (b)

(c) (d)

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(e) (f)

3.3 Summary of the Images Formed by a Concave Mirror

Position of object Position of image Size of image Nature of image 1. Between pole P and F 2. At focus (F) 3. Between F and C 4. At C 5. Beyond C 6. At infinity

Behind the mirror At infinity Beyond C

At C Between F and C

At focus (F)

Enlarged Highly enlarged

Enlarged Equal to object

Diminished Highly diminished

Virtual and erect Real and inverted Real and inverted Real and inverted Real and inverted Real and inverted

3.4 Uses of Concave Mirrors:

Concave mirrors are used as shaving mirrors to see a large image of the face.

Concave mirrors are used by dentists to see the large images of the teeth of patients.

Concave mirrors are used as reflectors in torches, vehicle head-lights and

search lights to get powerful beams of light. Concave mirrors are used as doctor’s head-mirrors to focus light coming

from a lamp on to the body parts of a patient (such as eye, ear, nose, throat, etc.) to be examined by the doctor.

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Concave dishes are used in TV dish antennas to receive TV signals from the distant communications satellites.

Large concave mirrors are used in the field of solar energy to focus sun’s rays for heating solar furnaces.

3.5 Sign convention for Spherical Mirrors

According to the New Cartesian Sign Convention:

(a) All the distances are measured from pole of the mirror as origin. (b) Distances measured in the same direction as that of incident light are taken as

positive. (c) Distances measured against the direction of incident light are taken as negative. (d) Distances measured upward and perpendicular to the principal axis are taken as

positive. (e) Distances measured downward and perpendicular to the principal axis are taken

as negative. The object is always placed on the left side of the mirror.

3.6 Mirror Formula

A formula which gives the relationship between image distance (v), object distance (u) and focal length (f) of a spherical mirror is known as the mirror formula.

1 1 1+ =Image distance Object distance Focal length

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1 1 1orv u f

where v = distance of image from mirror u = distance of object from mirror and f = focal length of the mirror

Linear Magnification Produced by Mirrors

The ratio of the height of image to the height of object is known as linear magnification.

Magnification = height of imageheight of object

or 2

1

hmh

where m = magnification h2 = height of image and h1 = height of object The linear magnification produced by a mirror is equal to the ratio of the image distance to the object distance, with a minus sign.

Magnification = image distanceobject distance

or vmu

where m = magnification v = image distance and u = object distance

3.7 Formation of image by a Convex Mirror

Whatever be the position of an object in front of a convex mirror, the image formed by a convex mirror is:

Behind the mirror Virtual and erect Diminished (smaller than the object)

When the distance of the object is changed from the convex mirror, then only the position and size of the image changes. There are two main positions of an object in case of a convex mirror:

(a) Anywhere between the pole (P) and infinity (b) at far-off distance called infinity.

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(a)

(b)

3.8 Summary of the Images Formed by a Convex Mirror

Position of object Position of image Size of image Nature of image 1. Anywhere between

pole P and infinity 2. At infinity

Behind the mirror between P and F

Behind the mirror at focus (F)

Diminished

Highly diminished

Virtual and erect

Virtual and erect

3.9 Uses of Convex Mirrors

Convex mirrors are used as rear-view mirrors in vehicles (like cars, trucks and buses) to see the traffic at the rear side (or back side).

Big convex mirrors are used as ‘shop security mirrors’.

Convex mirror as side view mirror

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3.10 How to Distinguish Between a Plane Mirror, a Concave Mirror and a Convex Mirror Without Touching Them:

We can distinguish between these mirrors just by looking into them, that is, by bringing our face close to each mirror, turn by turn. All of them will produce an image of our face but of different types. Following points briefly explain the image in different mirrors:

A plane mirror will produce an image of the same size as our face and we will look our normal self.

A concave mirror will produce a magnified image and our face will look much bigger.

A convex mirror will produce a diminished image and our face will look much smaller.

4. REFRACTION

The change in direction of light when it passes from one medium to another obliquely is called refraction of light. The bending of light when it goes from one medium to another obliquely is called refraction of light. The refraction (or bending) of light takes place at the boundary between the two media.

Refraction and reflection of light

Cause of Refraction: The refraction of light is due to the change in the speed of light on going from one medium to another. Thus, when light goes from one medium to another, its speed changes and this change in speed of light causes the refraction of light. Greater the difference in the speeds of light in the two media, greater will be the amount of refraction (or bending) of light.

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4.1 Optically Rarer Medium and Optically Denser Medium

A transparent substance in which light travels is known as a medium. Air, glass, certain plastics, water, kerosene, alcohol, etc. are all examples of medium.

A medium in which the speed of light is more, is known as optically rarer medium. Air is an optically rarer medium as compared to glass and water.

A medium in which the speed of light is less, is known as optically denser medium. Glass is an optically denser medium than air and water.

It has been found that: (i) When a ray of light goes from a rarer medium to a denser medium, it

bends towards the normal (at the point of incidence). For example: When a ray of light goes from air into glass, it bends towards the

normal. In this case, the angle of refraction (r) is smaller than the angle of incidence (i).

When a ray of light goes from air into water, it bends towards the normal.

(ii) When a ray of light goes from a denser medium to a rarer medium, it bends away from the normal (at the point of incidence). For example:

When a ray of light goes from glass into air, it bends away from the normal. In this case, the angle of refraction (r) is greater than the angle of incidence (i).

When a beam of light travelling in water enters into air, it bends away from the normal.

The Case of Light going from Air into Glass and Again into Air.

Refraction of light through a

rectangular glass slab

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4.2 The Case of Light Falling Normally (or Perpendicularly) on a Glass Slab

If the incident ray falls normally (or perpendicularly) to the surface of a glass slab, then there is no bending of the ray of light, and it goes straight. Since the incident ray goes along the normal to the surface, the angle of incidence in this case is zero (0) and the angle of refraction is also zero (0).

4.3 Effects of Refraction of Light

The refraction of light produces many effects which can be easily observed in our day to day life. It is due to the refraction of light that:

A stick (or pencil) held obliquely and partly immersed in water appears to be bent at the water surface. This apparent bending of the pencil is due to the refraction of light when it passes from water into air. An object placed under water appears

to be raised. For example: When a coin is under water then due to refraction of light, a virtual image of the coin is formed nearer to the water surface. And since the virtual image of coin which we see, is nearer to the water surface, so the coin appears to rise on adding water in the basin. A pool of water appears to be less deep than it actually is. This is

because when we look into a pool of water, we do not see the actual bottom of the pool, we see a virtual image of the bottom of the pool which is formed by the refraction of light coming from the pool water into the air. When a thick glass slab is placed over some printed matter, the letters

appear raised when viewed from the top. A lemon kept in water in a glass tumbler appears to be bigger than its

actual size, when viewed from the sides. The stars appear to twinkle on a clear night.

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4.4 Laws of Refraction of Light

1. According to the first law of refraction of light: The incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.

2. According to the Snell’s law of refraction of light: The ratio of sine of angle of incidence to the sine of angle of refraction is constant for a given pair of media (such as ‘air and glass’ or ‘air and water’). That is:

sine of angleof incidence = constantsine of angleof refraction

sinior = constantsin r

The value of the constant sinsin

ir

for a ray of

light passing from air into a particular medium is called the refractive index of that medium.

The refractive index is usually denoted by the symbol n. So:

Refractive index (n) = sinsin

ir

where sin i = sine of the angle of incidence (in air) and sin r = sine of the angle of refraction (in medium)

Refractive Index and Speed of Light

The refractive index of medium 2 with respect to medium 1 is equal to the ratio of speed of light in medium 1 to the speed of light in medium 2.

1 2

12medium medium

Speed of light in mediumnSpeed of light in medium

or 11 2

2

vnv

where 1 2n Refractive index of medium 2 w.r.t. medium 1 v1 = Speed of light in medium 1 and v2 = Speed of light in medium 2

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When light is going from one medium (other than vacuum or air) to another medium, then the value of refractive index is called relative refractive index.

When light is going from vacuum to another medium, then the value of refractive index is called the absolute refractive index.

The ratio of speed of light in vacuum to the speed of light in a medium, is called the refractive index of that medium. That is:

Refractive index of a medium Speed of light invacuumSpeed of light in medium

The refractive index depends on the nature of the material of the medium and on the wavelength (or colour) of the light used. If any two media are optically exactly the same, then no bending occurs when light passes from one medium to another. A substance having higher refractive index is optically denser than another substance having lower refractive index. Higher the refractive index of a substance, more it will change the direction of a beam of light passing through it. The optical density of a substance is different from its mass density.

5. SPHERICAL LENSES: (REFRACTION of LIGHT by SPHERICAL LENSES)

The working of a lens is based on the refraction of light rays when they pass through it. A lens is a piece of transparent glass bound by two spherical surfaces. There are two types of lenses: Convex lens and Concave lens.

(i) A convex lens is thick at the centre but thinner at the edges. (ii) A concave lens is thin in the middle but thicker at the edges.

Converging action of a convex lens Diverging action of a concave lens (CONVERGING LENS) (DIVERGING LENS)

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Important definitions related to Spherical Lenses

The centre point of a lens is known as its optical centre. The principal axis of a lens is a line passing through the optical centre of

the lens and perpendicular to both the faces of the lens. The principal focus of a convex lens is a point on its principal axis to

which light rays parallel to the principal axis converge after passing through the lens. A lens has two foci. The two foci of a lens are at equal distances from the optical centre, one on either side of lens.

The focal length of a lens is the distance between optical centre and principal focus of the lens.

5.1 Rules for obtaining Images formed by Spherical Lenses:

Rule 1. A ray of light from the object, parallel to the principal axis, after refraction from a convex lens, passes through the principal focus on the other side of the lens. In case of a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens.

Rule 2. A ray of light passing through a principal focus, after refraction from a convex lens, will emerge parallel to the principal axis. A ray of light appearing to meet at the principal focus of a concave lens, after refraction, will emerge parallel to the principal axis.

Rule 3. A ray of light passing through the optical centre of a lens will emerge without any deviation.

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5.2 Image formation in Convex lenses using Ray Diagrams

A convex lens is also known as a converging lens because it converges (brings to a point), a parallel beam of light rays passing through it. The type of image formed by a convex lens depends on the position of object in front of the lens. The object can be placed at different positions (or different distances) from a convex lens to get different types of images. For example, the object can be placed:

(a) between the optical centre (C) and focus (b) at the focus (c) between f and 2f (d) at 2f (e) beyond 2f (f) at infinity

(a) (b)

(c) (d)

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(e) (f)

5.3 Summary of the images formed by a Convex Lens

Position of object Position of image Size of image Nature of image 1. Between f and lens 2. At f (at focus) 3. Between f and 2f 4. At 2f 5. Beyond 2f 6. At infinity

On the same side as object At infinity Beyond 2f

At 2f Between f and 2f At f (at focus)

Enlarged Highly enlarged

Enlarged Same size as object

Diminished Highly diminished

Virtual and erect Real and inverted Real and inverted Real and inverted Real and inverted Real and inverted

5.4 Uses of Convex Lenses

Convex lenses are used in spectacles to correct the defect of vision called hypermetropia (or long sightedness).

Convex lens is used for making a simple camera. Convex lens is used as a magnifying glass (or magnifying

lens) (by palmists, watchmakers, etc.). Convex lenses are used in making microscopes,

telescopes and slide projectors (or film projectors).

5.5 Sign Convention For Spherical Lenses

These days New Cartesian Sign Convention is used for measuring the various distances in the ray diagrams of spherical lenses (convex lenses and concave lenses). According to the New Cartesian Sign Convention:

(a) All the distances are measured from the optical centre of the lens. (b) The distances measured in the same direction as that of incident light

are taken as positive. (c) The distances measured against the direction of incident light are taken

as negative.

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(d) The distances measured upward and perpendicular to the principal axis are taken as positive.

(e) The distances measured downward and perpendicular to the principal axis are taken as negative.

The object is always placed on the left side of the lens. On the basis of New Cartesian sign Convention, the focal length of a convex lens is considered positive. On the other hand, the focal length of a concave lens is considered negative.

5.6 Lens Formula

A formula which gives the relationship between image distance (v), object distance (u), and focal length (f) of a lens is known as the lens formula. The lens formula can be written as:

1 1 1v u f

where v = image distance u = object distance and f = focal length

Magnification Produced by Lenses The linear magnification is the ratio of the height of the image to the height of the object. That is:

Magnification = height of imageheight of object

or 2

1

hmh

where m = magnification h2 = height of image

and h1 = height of object The linear magnification produced by a lens is equal to the ratio of image distance to the object distance. That is:

Magnification = image distanceobject distance

or vmu

where m = magnification v = image distance and u = object distance

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5.7 Image formation in Concave lenses using Ray Diagrams

Whatever be the position of an object in front of a concave lens, the image formed by a concave lens is:

Virtual and erect Diminished (smaller than the object)

When the distance of the object is changed from the concave lens, then only the position and size of the image changes. There are two main positions of an object in case of a concave lens:

(a) Anywhere between the optical centre and infinity. (b) at infinity.

(a) (b)

5.8 Summary of the Images Formed by a Concave Lens

Position of object Position of image Size of image Nature of image 1. Anywhere between

centre (C) and infinity 2. At infinity

Behind optical centre (C) and focus (F) At focus (F)

Diminished Highly diminished

Virtual and erect

Virtual and erect

5.9 Uses of Concave Lenses

Concave lenses are used in spectacles to correct the defect of vision called myopia (or shortsightedness).

Concave lens is used as eye-lens in Galilean telescope. Concave lenses are used in combination with convex lens to make high

quality lens systems for optical instruments. Concave lens is used in wide-angle spyhole in doors.

6. POWER OF A LENS

The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it. The power of a lens depends on its focal length. The power of a lens is defined as the reciprocal of its focal length in metres.

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Power of a lens = 1( )focal lengthof thelens in metres

or 1Pf

where P = Power of the lens and f = focal length of the lens (in metres)

A lens of short focal length has more power whereas a lens of long focal length has less power. The unit of the power of a lens is dioptre. One dioptre is the power of a lens whose focal length is 1 metre. The power of a convex lens is positive. The power of a concave lens is negative.

6.1 Power of a Combination of Lenses

If a number of lenses are placed in close contact, then the power of the combination of lenses is equal to the algebraic sum of the powers of individual lenses. Thus, if two lenses of powers p1 and p2 are placed in contact with each other, then their resultant power P is given by:

P = p1 + p2

Image formation by a combination of two thin lenses in contact

In general, if a number of thin lenses having powers p1, p2, p3,…..etc, are placed in close contact with one another, then their resultant power P is given by:

P = p1 + p2 + p3 + ……….

7. TOTAL INTERNAL REFLECTION

When light travels from an optically denser medium to a rarer medium at the interface, it is partly reflected back into the same medium and partly refracted to the second medium. This reflection is called the internal reflection. When a ray of light enters from a denser medium to a rarer medium, it bends away from the normal, for example, the ray AO1B in given figure. The incident ray AO1

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is partially reflected (O1C) and partially transmitted (O1B) or refracted, the angle of refraction (r) being larger than the angle of incidence (i). As the angle of incidence increases, so does the angle of refraction, till for the ray AO3, the angle of refraction is π/2. The refracted ray is bent so much away from the normal that it grazes the surface at the interface between the two media. This is shown by the ray AO3D in given figure. If the angle of incidence is increased still further (e.g., the ray AO4), refraction is not possible, and the incident ray is totally reflected. This is called total internal reflection.

Refraction and internal reflection of rays from a point A in the

denser medium (water) incident at different angles at the interface with a rarer medium (air).

When light gets reflected by a surface, normally some fraction of it gets transmitted. The reflected ray, therefore, is always less intense than the incident ray, howsoever smooth the reflecting surface may be. In total internal reflection, on the other hand, no transmission of light takes place.

Observing total internal reflection in water with a laser beam (refraction due to glass of beaker neglected being very thin)

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7.1 Applications of Total Internal Reflection

Some of the technological applications of total internal reflection in nature are explained below:

(A) Mirage: On hot summer days, the air near the ground becomes hotter than the air at higher levels. The refractive index of air increases with its density. Hotter air is less dense, and has smaller refractive index than the cooler air. If the air currents are small, that is, the air is still, the optical density at different layers of air increases with height. As a result, light from a tall object such as a tree, passes through a medium whose refractive index decreases towards the ground. Thus, a ray of light from such an object successively bends away from the normal and undergoes total internal reflection, if the angle of incidence for the air near the ground exceeds the critical angle. The observer naturally assumes that light is being reflected from the ground, say, by a pool of water near the tall object. Such inverted images of distant tall objects cause an optical illusion to the observer. This phenomenon is called mirage.

(a) A tree is seen by an observer at its place when the air above

the ground is at uniform temperature

(b) When the layers of air close to the ground have varying temperature with hottest

layers near the ground, light from a distant tree may undergo total internal reflection, and the apparent image of the tree may create an illusion to the observer

that the tree is near a pool of water.

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(B) Diamond: Diamonds are known for their spectacular brilliance. Their brilliance is mainly due to the total internal reflection of light inside them. The critical angle for diamond-air interface (≅ 24.4°) is very small, therefore once light enters a diamond, it is very likely to undergo total internal reflection inside it. Diamonds found in nature rarely exhibit the brilliance for which they are known. It is the technical skill of a diamond cutter which makes diamonds to sparkle so brilliantly. By cutting the diamond suitably, multiple total internal reflections can be made to occur.

(C) Prism: Prisms designed to bend light by 90º or by 180º make use of total internal reflection. Such a prism is also used to invert images without changing their size. In the first two cases, the critical angle ic for the material of the prism must be less than 45º.

Prisms designed to bend rays by 90º and 180º or to invert image without changing its size make use of total internal reflection.

(D) Optical fibres: Now-a-days optical fibres are extensively used for transmitting audio and video signals through long distances. Optical fibres too make use of the phenomenon of total internal reflection. Optical fibres are fabricated with high quality composite glass/quartz fibres. Each fibre consists of a core and cladding. The refractive index of the material of the core is higher than that of the cladding. When a signal in the form of light is directed at one end of the fibre at a suitable angle, it undergoes repeated

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total internal reflections along the length of the fibre and finally comes out at the other end. Since light undergoes total internal reflection at each stage, there is no appreciable loss in the intensity of the light signal.

Light undergoes successive total internal reflections as it moves

through an optical fibre.

*******

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Class- 10th (Physics) Topic - LIGHT

Worksheet – 1

1. What is meant by reflection of light ? 2. State and explain the laws of reflection of light at a plane surface (like plane mirror), with the help of a labelled ray diagram. 3. What happens when a ray of light falls normally on the surface of plane mirror? 4. An incident ray makes an angle of 35° with the surface of a plane mirror. What is the angle of reflection ? 5. Write two points of difference between regular reflection and diffuse reflection. 6. Write three points of difference between a real image and a virtual image. 7. What is lateral inversion ? Explain by giving a suitable example. 8. Write the properties of an image formed by a plane mirror. 9. Shyam is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then find the distance between Shyam and his image. 10. If an object is placed at a distance of 10 cm in front of a plane mirror, how far would it be from its image ?

HOTS 11. How many images will be formed when two plane mirror are inclined at an angle of : (i) 30° (ii) 60° (iii) 90° (iv) 0° 12. A ray of light of frequency 5 × 1014 Hz is passed through a liquid. The wavelength of light measured inside the liquid is found to be 450 × 109 m. Calculate the refractive index of the liquid.

13. A light of wavelength 6000 Ao

in air, enters a medium with refractive index 15. What will be the wavelength of light in that medium ? 14. The refractive index of glass is 15 and that of water is 13. If the speed of light in water is 225 × 108 m/s, what is the speed of light in glass ?

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Class- 10th (Physics) Topic - LIGHT

Worksheet – 2

1. Define the principal focus of a concave mirror. 2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length ? 3. Name a mirror that can give an erect and enlarged image of an object. 4. Why do we prefer a convex mirror as a rear-view mirror, in vehicles ? 5. A convex mirror used for rear-view on an automobile has a radius of curvature of 3.00 m. If a bus is located at 5.00 m from this mirror, find the position, nature and size of the image. 6. An object, 4.0 cm in size, is placed at 25.0 cm in front of a concave mirror of focal length 15.0 cm. At what distance from the mirror should a screen be placed in order to obtain a sharp image? Find the nature and the size of the image. 7. Find the focal length of a convex mirror whose radius of curvature is 32 cm. 8. A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located ? 9. The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object ? (a) Between the principal focus and the centre of curvature (b) At the centre of curvature (c) Beyond the centre of curvature (d) Between the pole of the mirror and its principal focus. 10. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be (a) plane (b) concave (c) convex (d) either plane or convex. 11. We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror ? What is the nature of the image? Is the image larger or smaller than the object ? Draw a ray diagram to show the image formation in this case. 12. Name the type of mirror used in the following situations. (a) Headlights of a car. (b) Side/rear-view mirror of a vehicle. (c) Solar furnace.

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13. An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image. 14. The magnification produced by a plane mirror is + 1. What does this mean ? 15. An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size. 16. An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focused image can be obtained ? Find the size and the nature of the image. 17. Define the following terms : (i) Centre of curvature (ii) Radius of curvature (iii) Pole (iv) Principal Axis (v) Aperture 18. Write three uses each of concave mirror and convex mirror. 19. Draw ray diagrams to show the formation of images when the object is placed in front of a concave mirror (i) between its pole and focus (ii) between its centre of curvature and focus 20. What is the nature of image formed by a concave mirror if the magnification produced by the mirror is (a) + 4 (b) 2 ? 21. How will you distinguish between a plane mirror, a concave mirror and a convex mirror without touching them ? 22. An object is kept at a distance of 5cm in front of a convex mirror of focal length 10 cm. Calculate the position and magnification of the image and state its nature. 23. An object 3 cm high is placed at a distance of 8 cm from a concave mirror which produces a virtual image 45 cm high: (i) What is the focal length of the mirror ? (ii) What is the position of image ? (iii) Draw a ray diagram to show the formation of image . 24. An object 1cm tall is placed 30 cm in front of a convex mirror of focal length 20 cm. Find the size and position of the image formed by the convex mirror.

25. An object is kept in front of a concave mirror of focal length 20 cm. The image is three times the size of the object. Calculate two possible distances of the object from the mirror.

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26. An object is placed at a distance of 10cm from a convex mirror of focal length 15 cm. Find the position and nature of the image. 27. What is the nature of the image formed by a concave mirror if the magnification produced by the mirror + 3 ? 28. It is desired to obtain an erect image of an object using a concave mirror of focal length 20cm. (i) What should be the range of distance of the object from mirror ? (ii) Will the image be bigger or smaller than the object ? (iii) Draw a ray diagram to show the image formation in this case.

HOTS 29. The apparent depth of an object at the bottom of tank filled with a liquid of refractive index 13 is 77 cm. What is the actual depth of the liquid in the tank ? 30. The velocity of light in glass is 2 × 108 m/s and that in air is 3 × 108 m/s. By how much would an ink dot appear to be raised, when covered by a glass plate of 6 cm thickness ? 31. A small pin fixed on a table top is viewed from above from a distance of 50 cm. By what distance would the pin appear to be raised if it is viewed from the same point through a 15cm thick glass slab held parallel to the table? Refractive index of glass = 15. Does the answer depend on the location of the slab ? 32. A mark is made on the bottom of a beaker and a microscope is focussed on it. The microscope is raised through 15cm. To what height water must be poured into the beaker to bring the mark again into focus? Give that refractive index for water is 4/3. 33. A vessel contains water upto a height of 20 cm and above it an oil upto another 20 cm. The refractive indices of water and oil are 133 and 130 respectively. Find the apparent depth of vessel when viewed from above.

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Class- 10th (Physics) Topic - LIGHT

Worksheet – 3

1. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal ? Why ? 2. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 × 108 m s1. 3. Find out from given Table, the medium having optical density. Also find the medium with lowest optical density.

4. You are given kerosene, turpentine and water. In which of these does the light travel faster ? Use the information given in above Table. 5. The refractive index of diamond is 2.42. What is the meaning of this statement ? 6. What is meant by refraction of light ? Draw a labelled ray diagram to show the refraction of light. 7. Explain why, a stick half immersed in water appears to be bent at the surface. Draw a labelled diagram to illustrate your answer. 8. (a) With the help of a diagram, show how when light falls obliquely on the side of a rectangular glass slab, the emergent ray is parallel to the incident ray. (b) Show the lateral displacement of the ray on the diagram. (c) State two factors on which the lateral displacement of emergent ray depends. 9. State and explain the laws of refraction of light with the help of a labelled diagram.

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10. What is the speed of light in a medium of refractive index 65

if its speed in air

is 3 × 105 km/s ? 11. If the refractive index of water for light going from air to water be 133, what will be the refractive index for light going from water to air? 12. Light travels more quickly through water than through glass. Which is optically denser - water or glass ?

HOTS 13. What is totally reflecting prism ? How can it be used to: (i) deviate a ray through 900. (ii) deviate a ray through 1800, and (iii) invert an image without the deviation of the ray. 14. Find the value of critical angle for a material of refractive index 3 . 15. Calculate the speed of light in a medium whose critical angle is 45°. 16. The velocity of light in a liquid is 15 × 108 m/s and in air, it is 3 × 108 m/s. If a ray of light passes from this liquid into air, calculate the value of critical angle. 17. What is the small index of refraction of the material of a right-angled prism with equal sides for which a ray of light entering one of the sides normally will be totally reflected ?

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Class- 10th (Physics) Topic - LIGHT

Worksheet – 4

1. A concave lens has focal length of 15 cm. At what distance should the object from the lens be placed so that is forms an image at 10 cm from the lens? Also, find the magnification produced by the lens ? 2. A 2.0 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 10cm. The distance of the object from the lens is 15 cm. Find the nature, position and size of the image. Also find its magnification. 3. Define 1 dioptre of power of a lens. 4. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens. 5. Find the power of a concave lens of focal length 2 m. 6. Which one of the following materials cannot be used to make a lens ? (a) Water (b) Glass (c) Plastic (d) Clay 7. The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object ? (a) Between the principal focus and the centre of curvature (b) At the centre of curvature (c) Beyond the centre of curvature (d) Between the pole of the mirror and its principal focus. 8. A spherical mirror and a thin spherical lens have each a focal length of 15 cm. The mirror and the lens are likely to be: (a) Both concave.

(b) Both convex. (c) the mirror is concave and the lens is convex. (d) the mirror is convex, but the lens is concave.

9. Which of the following lenses would you prefer to use while reading small letters found in a dictionary ? (a) A convex lens of focal length 50 cm. (b) A concave lens of focal length 50 cm. (c) A convex lens of focal length 5 cm. (d) A concave lens of focal length 5 cm.

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10. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object ? Verify your answer experimentally. Explain your observations. 11. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the observations. 12. A concave lens of focal length 15cm forms an image 10 cm from the lens. How far is the object placed from the lens ? Draw the ray diagram. 13. Find the focal length of a lens of power 2.0 D. What type of lens is this? 14. A doctor has prescribed a corrective lens of power + 1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging ? 15. What is a lens? Distinguish between a convex lens and a concave lens. 16. Name two factors on which the focal length of a lens depends. 17. Where should an object be placed in order to use a convex lens as a magnifying glass ? 18. A ray of light is going towards the focus of a concave lens. Draw a ray diagram to show the path of this ray of light after refraction through the lens. 19. An object is placed 10cm from a lens of focal length 5cm. Draw the ray diagrams to show the formation of image if the lens is : (i) converging (ii) diverging 20. A concave lens of 20cm focal length forms an image 15cm from the lens. Compute the object distance. 21. A thin lens has a focal length of 25 cm. What is the power of the lens and what is its nature ? 22. Two thin lenses of power, + 45 D and 25 D are placed in contact. Find the power and focal length of the lens combination. 23. A 2.0cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 10cm. If the object distance is 15cm, find the nature, position and size of the image. Also find its magnification. 24. A compound lens is made of two lenses in contact having powers +125 D and 25 D. An object is placed at 15cm from this compound lens. Find the position and nature of the image formed.

25. Given the refractive index of water and glass is 4/3 and 3/2 respectively. Write the relation and find the value of refractive index of water with respect to glass and glass with respect to water.

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26. Find the net focal length of a combination of lenses whose focal lengths are 15 cm and 5 cm respectively.

HOTS

27. Light from a point source in air falls on a convex spherical glass surface ( = 15, radius of curvature = 20cm). The distance of light source from the glass surface is 100 cm. At what position is the image formed ? 28. A convex refracting surface of radius of curvature 20cm separates two media of refractive indices 4/3 and 160. An object is placed in the first medium ( = 4/3) at a distance of 200 cm from the refracting surface. Calculate the position of the image formed. 29. The radius of curvature of each face of biconcave lens, made of glass of refractive index 15 is 30 cm. Calculate the focal length of the lens is air. 30. Double convex lenses are to be manufactured from a glass of refractive index 155, with both faces of the same radius of curvature. What is the radius of curvature required if the focal length of the lens is to be 20 cm ? 31. The radii of curvature of the faces of a double convex lens are 10 cm and 15 cm. If focal length is 12 cm, what is the refractive index of glass? 32. A biconvex lens has a focal length half the radius of curvature of either surface. What is the refractive index of lens material? 33. The radius of curvature of each surface of a convex lens of refractive index 15 is 40 cm. Calculate its power. 34. Two thin lenses, when in contact, produce a combination of power 10 dioptres. When they are 025 m apart, the power reduces to +6 dioptres. Find the focal length of the lenses.

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