Light and Optics

Form 4 Physics (SPM) – Chapter 5

Light Electromagnetic radiation that is perceived by the human eye Has two forms

Waveform Particleform

Light travels in the form of rays in a straight line Light rays tend to spread away from the source

Wave properties of light Reflection Refraction Diffraction Interference

Light is a form of energy Visible light is made up of 7 colours of various intensities Light, like any electromagnetic radiation travels through

vacuum at a speed of approximately 3X108 ms-1

Optics Study of interaction of light with objects that reflect or scatter

light Light is able to

Completely penetrate transparent objects Partially penetrate translucent objects Not penetrate opaque objects

Light rays

Key notes Object – material that emits light rays Image – representation of object after light has reflected or

refracted Ray diagram – diagram showing path of light through reflection or

refraction Line of sight – order or position of the eye to view the image formed Solid lines – real light rays Dotted lines – virtual light rays Convergence – meeting of light rays at a point Divergence – spreading of light rays from a point Focus point – point where light rays are converged or diverged

Reflection Occurs when the direction of light changes when it strikes an

opaque material Light ray before reflection – incident ray Light ray after reflection – reflected ray Imaginary line that is perpendicular to surface – normal line Angle of incidence, i – angle between incident ray and normal

line Angled of reflection, r – angle between reflected ray and

normal line

Incident ray Reflected ray

Normal

Plane

i r

Two types of reflection Specular reflection

Occurs when light strikes a smooth and shiny surface Diffused reflection

Occurs when light strikes an uneven surface

Law of reflection Incident, reflected ray and normal line lies on the same plane i = r

Path of light reflection is represented in a ray diagram

Key terms in a ray diagrams C – centre of curvature of mirrors or optical centre of lenses F – focus point of curved mirrors or lenses f – focal length (distance of F from C) u – distance of object to the surface of reflection or refraction v – distance of image to the surface of reflection or refraction Principle axis, P – light ray that is perpendicular to the surface

of reflection or refraction and crosses through C and F

* In concave and convex mirrors, C = 2F

Reflection on a plane mirror

Image

Object

Line of sight

Plane mirroru

v

Properties of image formed Same size as object v = u Laterally inverted

Flipped horizontally Left of Object becomes right of Image

Virtual Image exists within the mirror, a.k.a. another plane The image cannot be formed on a screen

Reflection on a concave mirror

..F C

P

Reflection on a convex mirror

. .F C

P

A concave mirror is also called a converging mirror A convex mirror is also called a diverging mirror The curvier the mirror, the smaller the focal length When incident rays are parallel to P, then reflected rays will

pass through F When incident rays passes through F, then reflected rays will

be parallel to P When incident rays passes through C, then reflected rays will

pass through C in the opposite direction, parallel to the incident ray

Properties of image in concave and convex mirrors

Location of objectProperties of concave

imageProperties of convex

image

u > C Real, Diminished, Inverted

Virtual, Diminished, Upright

u = CReal, Same size as object,

Inverted

C < u < F Real, Magnified, Inverted

u = F Formed at infinity

u < F Virtual, Magnified, Upright

Applications of reflection Parallax mirror in measuring instruments Wing and rear view mirrors Periscope Vanity mirror Satellite dish Headlights and torchlight reflectors

Refraction Phenomenon where the speed of light changes as it

propagates from one medium to another The change of speed causes a change in the direction of

propagation When light propagates from a medium of low density to a

medium of high density, its speed decreases, causing the direction of propagation to approach the normal

The opposite is true when light passes from a medium of high density to a medium of low density

Incident ray – i Refracted ray – r When light travels from a medium of low density to a medium of

high density: Its speed decreases Its direction changes i > r

When light travels from a medium of high density to a medium of low density: Its speed increases Its direction changes i < r

r

r i

i

i > r

High density

High density

Low density

Low density

i < r

Refractive Law The incident ray, refracted ray and the normal line all lie on the same plane The ratio of sin i to sin r yields a constant known as the refractive index

sin i = n, where n = refractive index sin r

Refractive Index, n Has no units Indicates the light bending ability of a medium Value equals to the ratio of sin i to sin r Value also equals to ratio of speed of light in vacuum to

speed of light in medium Value also equals to ratio of real depth to apparent depth

Total internal reflection Is a form of light refraction. Occurs when light travels from a medium of high density to a

medium of low density, where i > r. Occurs when the i is very large causing the r to be more than

90˚. Critical angle, c is the value of i that results in r = 90˚. When i > c, total internal reflection occurs and the reflected

ray is present in the same medium as the incident ray.

r > 90˚

i > cHigh density

Low density

Total internal reflection

r = 90˚

i = c

Critical angle, c

High density

Low density

Observations and applications of refraction and total internal reflection Sunset below the horizon Rainbow formation Mirages Fish’s eye view Fibre optics Prism periscope Prism binoculars Perfectly cut diamond

Lenses

Bi

Bi

Refraction through a biconvex lens

. . . .F C F P

F = Focal pointC = Optical centreP = Principle axisf = focal length

ff

Refraction through a biconcave lens

. . . .F C F P

F = Focal pointC = Optical centreP = Principle axisf = focal length

ff

A concave lens is also called a diverging lens. A convex lens is also called a converging lens. The larger the lens, the larger the f value. The thicker the lens, the smaller the f value. When incident rays are parallel to P, then refracted rays will

pass through F When incident rays passes through F, then refracted rays will

be parallel to P No refraction occurs when incident rays passes through C. The

rays simply pass through the lens in a straight line

Properties of image in convex and concave lenses

Location of objectProperties of convex

imageProperties of concave

image

u = ∞ Real, Diminished, Inverted

Virtual, Diminished, Upright

u > 2f Real, Diminished, Inverted

u = 2fReal, Same size as object,

Inverted

2f < u < f Real, Magnified, Inverted

u = f Image is formed at infinity

u < f Virtual, Magnified, Upright

Value of f Positive – convex lens Negative – concave lens

Value of u and v Positive – real image Negative – virtual image

Lens law 1/f = 1/u + 1/v f = focal length u = object length v = image length

Linear magnification, m m = v/u m = hi/ho, where hi = height of image and ho = height of object

Power of lens, P P = 1/f Unit = m-1 or Diopter (D)

Magnifying glass

. . . .F C F P

Compound Microscope

Fo Fo Fe Fe

. . . .

Objective lens

Eyepiece lens Construction line

Image is:•Magnified•Inverted•Virtual

Telescope

Fo Fo / Fe Fe

. . .

Objective lens

Eyepiece lens Construction line

Image is formed at infinity