Physics 504

Chapters 1 & 2Opticks

Alexander Pope

•Nature and Nature’s laws lay hid in night;

•God said, Let Newton be! •And all was light.

•Epitaph intended for Sir Isaac Newton

Light

• Light is an electromagnetic disturbance whose propagation allows the transportation of light energy.

• Light is considered to travel as a wave AND as particle or photon.

• As a wave, light has different frequencies or cycles per second (cps)

• The unit of frequency is a hertz (Hz).

Transmission of light• Recti-Linear propagation: light travels in straight

lines;• Ray: the path taken by light energy;• Ray diagrams: pictures that show the paths of

light rays;• Diverge: light rays become farther apart;• Converge: light rays come closer together;• Parallel rays: rays that travel the same distance

apart;• Speed of light: c = 3x108 m/s

Speed of light

• Light travels at a definite speed (c);• c = 3x108 m/s = 300000 km/s• Distance = speed x time• Calculate the distance (km) travelled in:

• 1 minute• 1 hour • 1 day • 1 year - 365.25 days – 1 lightyear

Pinhole Camera Image

Pinhole camera

• Pinhole cameras demonstrate that light travels in straight lines;

• An image is formed inverted vertically and horizontally on the back of the

camera• The image will be inverted,smaller and real.

Pinhole camera• Magnification = height of image

height of object

• M = h i/ h o

• As well, M = distance to image

distance to object

• Or, M = d I/d O

• So h i/ h o = d I/d O

QuestionA pinhole camera is used to show the image of a turtle on its screen as shown below.

Screen

Pinhole camera

Which of the following is the image of the turtle that will appear on the screen of the pinhole camera?

A)

C)

B)

D)

Question

Alex places a 5.00 cm tall candle 30.0 cm in front of a pinhole camera that has a length of 50.0 cm.What is the magnification of the candle?

A) 1.66

B) 0.166

C) 0.63

D) 0.1

Shadow• Umbra: the area of total

shadow – this area receives no light;

• Penumbra: the area of

partial shadow – this area

receives some light;

• E.g. lunar/solar eclipses

Question

When only the Moon's penumbral shadow strikes Earth, we see a partial eclipse of the Sun from that region.

Sun Moon Earth

A B C

In which region(s) on Earth do we see a partial eclipse of the Sun?

A)

Region A only

B)

Region B only

C)

Regions A and C

D)

Regions A, B and C

Activity

• Page 35, Q. 2,6,7,8,9• Page 40, Q. 1-3, 5

Summary• A luminous light source emits its own light.

• A luminous light source can produce its light by incandescence, luminescence, fluorescence, or phosphorescence.

• Light striking an object can be reflected, refracted, absorbed or any combo of the 3.

• Other light phenomena: diffraction, diffusion, scattering, interference, dispersion.

• Light travels in straight line ray paths – rectilinear propagation.

Summary

• When an object creates a shadow, the area of total shadow is the umbra, the area of partial shadow is the penumbra.

• For a pinhole camera, M, the magnification is given by

• M = hi/ho = di/do.

Physics 504

Reflections on Light

Vocabulary

• Reflection in a plane mirror, • Incident ray – ray coming into

surface• Point of incidence – where the ray

hits the surface• Reflected ray – the reflected ray• Normal – line at 90 ⁰ to point of

incidence.

Vocabulary

• Image Type • Virtual only appears in the eye of the

beholder• Real can be put on a screen

• Attitude or Orientation – upright or inverted (upside down)

• Size – smaller, same, bigger

Types of Reflection

• Diffuse reflection occurs when the reflected rays of light are not parallel.

• The surface is usually rough and uneven.

• E.g. paper, snow

Types of Reflection

• Specular (regular) reflection occurs when the reflected rays of light emerge parallel from the surface.

• The surface is usually smooth and polished.

• E.g. mirror, glass

Rays

Incident Rays Reflected Rays

normal normal

Angle of incidence Angle of

reflection

Angle of Incidence & Reflection• The angles of incidence and reflection

are measured relative to the NORMAL –

• NOT THE SURFACE.• The angle of incidence equals the

angle of reflection.• The incident ray, the reflected ray and

the normal are all in the same plane.

Formation of Image by a Plane Mirror – Ray Diagram• A ray of light incident on a plane mirror at 90o

gets reflected from the mirror along the same path.

• A ray of light falling on a plane mirror at any angle gets reflected from the mirror such that the angle of incidence is equal to the angle of reflection.

• P. 45, Q. 1, 3

Images formed in a Plane Mirror• A plane mirror is a flat mirror.• Images are the same attitude – upright or

inverted.• Images are the same distance “behind”

the mirror as the object is in front of it.• Images formed are virtual – they cannot

be projected on a screen.• A line joining the image and object is

perpendicular to the plane mirror.

QuestionQuestion

An object is placed in front of a plane mirror. Which of the following diagrams correctly represents the image formed in the plane mirror?

A)

C)

B)

D)

QuestionQuestion

An object is placed in front of a plane mirror. Which statement correctly describes the characteristics of the image?

A)

The image is real, upright, smaller than the object and located in front of the mirror.

B)

The image is virtual, inverted, larger than the object and located behind the mirror.

C)

The image is real, inverted, the same size as the object and located in front of the mirror.

D)

The image is virtual, upright, the same size as the object and located behind the mirror.

Exercise• Using a light beam, ruler and plane

mirror, draw the incident, normal and reflected ray coming off the mirror.

• Compare the angles.

• __________________________________

Field of Vision

• The field of vision from a mirror is determined by the width of the mirror, the distance of the observer to the mirror and its curvature.

• In stores curved mirrors are used to give a wider field of vision.

FIELD OF VISION

normal normal

Field of Vision

Observer

ΘrΘi Θr Θi

Plane mirror

Field of Vision

• Our view in a mirror depends on how we position ourselves in front or to the side of the mirror.

• The field of vision can be determined through ray diagrams:

• a. Draw two normals, one at either end of the mirror,

• b. Draw an incident ray from your eye to each normal.

• c. Draw a reflected ray from each normal.• d. Remember the laws of reflection when drawing

both sets of rays (i = r)• Make a diagram – ask teacher.

Activity: Determine Field of Activity: Determine Field of Vision: Vision: for a Plane Mirrorfor a Plane Mirror ________MirrorMirror__________

OO

Activity: Determine Field of Activity: Determine Field of Vision: Vision: For a Curved MirrorFor a Curved Mirror

OO

QuestionQuestionAn observer is standing in front of a reflective window in which he can see the images of some shrubs.

REFLECTIVE WINDOW

I

II

III

IV

OBSERVER

Based on the above diagram, the images of which shrubs can be seen by the observer?

A)

III only

B)

II and III only

C)

III and IV only

D)

I, II, III and IV

Good, now answer: p. 47, Q. 1-3

34

Concave Mirror

35

The Inside of A Spoon

• Why is it when you look into a soup spoon that your image turns upside down sometimes?

• The inside of a spoon is a concave mirror. • If the inside of the spoon is held close to the

eye, a magnified upright view of the eye will be seen (in this case the eye is closer than the focal point of the mirror).

• If the spoon is moved farther away, a smaller upside-down view of the whole face will be seen.

36

Concave Mirrors

• The concave mirror has a reflection surface that curves inward, like a portion of the interior of a sphere.

• When light rays that are parallel to the principal or optical axis reflect from the surface of a concave mirror, they converge on the focal point (black dot) in front of the mirror.

37

Converging Mirror

• Concave mirrors bring light rays to a focus and are called converging mirrors.

38

Rules for Con/Di-Verging Mirrors• A ray that is parallel to the principal axis is

reflected through the principal focus – real or virtual.

• A ray passing through the principal focus is reflected parallel to the principal axis.

• A ray passing through the centre of curvature is reflected back along the same path.

• The centre of curvature or radius is twice the focal length.

• C = 2f.

QuestionQuestionLook at the diagram below. A 12 cm tall object is placed in front of a concave mirror. The focal length is 30 cm. The object is located 70 cm from the top of the mirror.

Object

C F

What will be the height of the image reflected by the mirror?

A) 5.1 cm

B)

9.0 cm

C)

16 cm

D)

28 cm

Concave MirrorsObject’s Position

Image Characteristics

Type Orientation

Size Position

At ∞ Real Point Image At F

Beyond C Real Inverted Smaller Between F and C

At C Real Inverted Same size At C

Between C and F

Real Inverted Bigger Beyond C

At F No Image

Between F and V

Virtual Upright Larger than the object

Between F and V behind the mirror, farther from the mirror than the object.

41

Convex Mirrors• The convex mirror has a reflecting surface

that curves outward like a portion of the exterior of a sphere.

• Light rays parallel to the optical axis are reflected from the surface in a manner that diverges from the focal point, which is behind the mirror.

• Images formed with convex mirrors are always right side up and reduced in size.

• These images are also termed virtual images, because they occur where reflected rays appear to diverge from a focal point behind the mirror.

42

Diverging Mirrors

• Diverging mirrors are convex.• Regardless of the position of the object

reflected by a convex mirror, the image formed is always virtual, upright, and reduced in size.

• When the brain retraces the rays, they appear to come from behind the mirror where they would converge, producing a smaller upright image

• The image is upright since the virtual image is formed before the rays have crossed the focal point.

Convex Mirrors

Object’s Position

Image Characteristics

Type Orientation

Size Position

Any position

Virtual

Upright Smaller than the object

Between F and V behind the mirror, closer to the mirror than the object.

Exam Question – on board

 A)

 + 1.60 

B) + 0.63 

C) - 0.63 

D) - 1.63

The teacher will draw this on the board, please.

Alex places a 5.00 cm tall candle 30.0 cm in front of a mirror that has a focal length of ‑50.0 cm.What is the magnification of the candle?

Good, now do, page 54, Q. 4, 7, 9

Mirror Equations

• do is the distance to the object

• di is the distance to the image

• f is the focal length

• hi is the image height

• ho is the object height

• N.B. the negative sign

Conventions for the Equations

• Distances are measured from the vertex.• Focal lengths are positive for converging mirrors

and negative for diverging mirrors.• Radii of curvature are positive for converging

mirrors and negative for d.m.• Image and object distances are positive for real

images and objects.• Image and object distances are negative for

virtual images and objects.• Image and object heights are positive when

upright and negative when inverted.

Exam Question

 A)

 20.0 cm

 C)

 90.0 cm

 B)

 60.0 cm

 D)

 120.0 cm

An object, 45.0 cm high, is placed in front of a convex mirror. A virtual image, 15.0 cm high, is formed.The focal length of the mirror is -30.0 cm.

At what distance is the object from the mirror?

Good, now do: p. 70, Q. 1,2,9,10 and p. 76, Q. 1,2,9

48

Uses for Concave Mirrors

• Concave mirrors are used in optical telescopes to collect the faint light emitted from very distant stars.

• The curved surface concentrates parallel rays from a great distance into a single point for enhanced intensity.

• This mirror design is also commonly found in shaving or cosmetic mirrors where the reflected light produces a magnified image of the face.

49

Uses for Convex Mirrors• Convex mirrors are often used in automobile

right-hand rear-view applications where the outward mirror curvature produces a smaller, more panoramic view of events occurring behind the vehicle.

• Convex mirrors are also used as wide-angle mirrors in hallways and businesses for security and safety.

• Carnival Fun House mirrors often incorporate a mixture of concave and convex surfaces, or surfaces that gently change curvature, to produce bizarre, distorted reflections when people observe themselves.

Other Reflections

• Non-optical reflection can occur as:• Echoes – sounds reflected from

surface;• Radio and microwaves;• Ocean waves when they recede;• Strings/Springs – as in guitar

strings;

Summary

• A normal is a line drawn at right angles to a reflective surface at the point of incidence.

• The Laws of Reflection are:– The incident ray, the reflected ray &

the normal are all in the same plane;– The angle of incidence equals the

angle of reflection

• Specular reflection is the reflection of light

from smooth plane surfaces. Rays parallel.• Diffuse reflection is the reflection of light

from irregular surfaces. Rays not parallel.• The field of vision using a plane mirror

depends on the size of the mirror and the distance of the observer and the object-to-be-seen from the mirror.

• Curved mirror terms – centre of curvature, principal axis, vertex, radius of curvature, principal focus, focal plane & focal length.

• All rays parallel to each other and striking a converging mirror meet at the focal point.

• The focal length is the distance between the principal focus and the vertex.

• In a curved mirror, the focal length is ½ the radius.

• Rays from distant objects are considered parallel.

• Spherical aberration occurs in curved mirrors when parallel rays do not meet at the focal point. A parabola can solve this.

• The Laws of Reflection apply to non-optical phenomena, like sound.

• Lenses are used to correct vision problems.