Images and Plane Mirrors

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DIAGNOSTIC /REMEDIAL TEST

02 IMAGES AND PLANE MIRRORS

This test is one of a series in Introductory Physics made available on the Website of the

School of Physics, Monash University, Australia

(www. physi cs. monash. edu. au/ communi t y).This test is NOT for the purposes of assessment. It is to assist you in locating

misconceptions and misunderstandings and generally to assist you in your study of

Physics. You should work by yourself and at your own pace following the directions

given. It is not necessary to attempt the test all at once. You may like to do it bit-by-bit,

waiting until you have covered a particular topic in class or in your reading of your textbook or you may like to "plunge in " before you begin your study of the topic.

Questions are on the left hand (even-numbered) pages. While reading or working on

these,keep the right hand (odd-numbered) answer page covered. DO NOT PEEK

AT THE ANSWERS ON THE RIGHT HAND PAGE !

The test was compiled by and largely authored by Emeritus Professor Bill Rachinger

who would appreciate any comments or suggestions for improvement. These could be

sent to him at bi l l . r achi nger@sci . monash. edu. auorEmeritus Professor Bill Rachinger, School of Physics

Monash University, P.O.Box 27, Vic 3800 Australia

Diagrams were produced by

Mr Steve McCausland, formerly of Department of Physics, Monash University


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COVER THE RIGHT HAND (ODD-NUMBERED) PAGES

DO NOT PEEK!

1. In which of the following objects is light made i.e. which is a source of light.

A. A candle flameB. A screen on which a movie is being shown.

C. The sun.

D. The screen of a switched-on T.V.

E. A torch (flashlight) globe.

F. A mirror.

G. A poster on your bedroom wall.

In the space below write the letter corresponding to the correct answer(s) and below this the

reason why the others are not sources of light.

ANS.

Why are the other answers not light sources?

GO STRAIGHT TO THE NEXT QUESTION. DON'T CHECK YOUR ANSWER YET.

WAIT UNTIL YOU ARE INSTRUCTED TO DO SO.

2. After you light a candle in a previously darkened room you see both the flame and the wax

candle. Which of the following do you consider the best reason for being able to see the wax

candle?

A. Light emitted by the flame is reflected by your eyes and strikes the candle.

B. Light from the flame is conducted down the inside of the wax candle. Some of this leaks

out of the candle and reaches your eyes.

C. Light from the flame fills the room and is reflected by the various objects in the room.

Some is reflected by the candle and reaches your eyes.

D. The heat from the candle causes the wax to become warm and glow and some of the light

from this reaches your eyes.

In the space below write the letter corresponding to the correct answer(s) and below this thereason why the other answers are wrong.

ANS.

Comment on the other answers.

GO STRAIGHT TO THE NEXT QUESTION. DON'T CHECK YOUR ANSWER YET.

WAIT UNTIL YOU ARE INSTRUCTED TO DO SO.


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1. A,C,D and E are sources of light or luminous objects. Light is produced within these

objects.

The others are objects which are seen when light from some luminous object such as the sun is

reflected from them and enters your eyes. A mirror or a poster cannot be seen in a darkened

room and the movie screen and the image on it are seen by the reflection of the light comingfrom the movie projector.

2. C is correct. The wax body of the candle is seen because light is reflected from it. Light

from the candle is reflected, maybe many times by the walls and objects in the room. Some will

be reflected by the candle and enter your eyes.

A is incorrect. "Seeing" involves light travelling from the candle to your eyes.

B is incorrect. Light does not travel (to any large extent) through a wax candle. You

cannot see through a wax candle. It is opaque.

D is incorrect. Warm wax is not a source of light. It does not glow. If you blew out the

candle flame you would not see the wax candle although the wax would still be warm.


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3. Consider two situations in which a movie is being viewed:

(i) on a movie screen in a darkened cinema

(ii) on a T.V. screen in a darkened room.

There are certain propositions (A-H) listed below which may relate to one or both of

these situations.

Which of these relate to the cinema screen?___________ Give your reasons for yourchoice and comment on the other answers.

Which of these relate to the T.V.screen?____________ Give your reasons for your

choice and comment on the other answers.

A. Light is being made on the screen.

B. Light from the screen is reaching your eyes.

C. Light is being reflected from the screen.

D. The dark parts of the image on the screen correspond to places where light is hitting the

screen and being absorbed.

E. No light is reaching your eyes from the dark parts of the image on the screen.

F. The bright parts of the image on the screen correspond to places where light is striking the

screen.

G. The bright parts of the image correspond to places where light being sent out by our eyes is

being reflected.

H. The bright parts of the screen correspond to places where light is being made.

CHECK YOUR ANSWERS TO THE LAST GROUP OF QUESTIONS. REFER TO THE

APPROPRIATE SECTIONS OF YOUR TEXT BOOK FOR FURTHER INFORMATION.

4. When you see your image in a mirror, where is the image located?

A. Floating in front of the mirror.

B. On the mirror surface (the silvered layer at the back of the glass).

C. Behind the mirror surface.

D. In your eye.

ANS__________

CHECK YOUR ANSWERS TO THIS QUESTION. REFER TO THE APPROPRIATE

SECTIONS OF YOUR TEXT BOOK FOR FURTHER INFORMATION.


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3. For the cinema screen: B,C,E and F are appropriate.

Light from the movie projector forms an image of the frames of the movie film on the screen.

Light forming the bright parts of the image is reflected off the screen (C,F) and some of this

enters your eyes (B).

The dark spots on the screen correspond to places where there is no light from the projectorreaching the screen (It has been blocked by the dark areas in the movie film).

Light is not absorbed at the dark spots on the screen as suggested in D. Thus the eye receives

no light from the dark spots (E). This is why they are perceived as "dark".

G is untrue. Light is not sent out by our eyes. H is untrue. All the light from the bright spots

has come from the projector and has been reflected from these spots. The movie screen is not

"luminous".

For the T.V. screen: A,B,E and H are appropriate.

Whereas the movie screen and the image on it are seen by light coming from the projector and

reflected by the screen, the T.V. screen is self-luminous.Light is being made where electrons strike the coating on the screen (A). These are the bright

spots in the image (H) and the light from these enters your eyes (B).

No light is being made at the dark spots and so no light is sent from them to your eyes (E).

G is untrue. Light is not sent out by our eyes. C,D and F are incorrect. Light is not falling on

the screen to be reflected or absorbed. The bright spots are self-luminous,

4. C is correct. This may seem surprising. Many people believe that the image is on the mirror

surface just behind the glass, like a painting or photograph. Question 6 will discuss this

problem.


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5. When light strikes a mirror it is reflected. What happens to light when it strikes a sheet of

white paper?

A. It is reflected.

B. It is stopped by the paper which then glows.

C. It stays in the vicinity of the paper creating a glowing white aura around it.

D. It is absorbed by the paper causing it to light up and appear white.In the space below write the letter corresponding to the correct answer(s) and illustrate this if

appropriate with a diagram. Below this write reasons why the other answers are wrong.

ANS.

Why are the other answers wrong?




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5. A is correct. The main difference between a mirror and a sheet of paper is in its surface

roughness.

A mirror has a very flat surface and a beam of light is reflected in a very orderly way. It is this

which allows you to see nice sharp images in a mirror.

The fibres which constitute paper give it a very rough surface and light falling on the paper is

reflected off in all directions. You do not see sharp images produced by a piece of paper, only a

white blur.

D is incorrect. White paper absorbs very little light . Black paper absorbs a lot. That is

why it appears black!

B and C are incorrect. The paper is not (to any large extent) self-luminous. It is largely

a reflector of light. Light neither stops nor remains in the vicinity of the paper. It travels athigh speed.

Fig.1

Fig.2


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6. Your friend tells you that you have a red spot on your cheek just below your eye. To

observe it you use a mirror. Let's see how this mirror works by studying what happens to

light coming from the red spot and striking the mirror.

Figure 3. above shows the positions of the red spot, your eye and the mirror. It also shows two

rays (or paths) of light coming from the red spot and striking the mirror. These are reflected

from the mirror according to the simple law of reflection which states that the angle ofincidence equals the angle of reflection as shown in Figure.4.

Using this law you should draw in on

Fig3 the paths of the two light rays after

reflection from the mirror. (You may find the

square grid useful in getting the angles correct.)

Now produce these lines backwards into

the space behind the mirror (Draw these lines

dotted). Locate the point where these two lines

meet. The image of the red spot is located at thispoint. How is its position related to the position

of the real red spot?

CHECK YOUR ANSWER TO THIS QUESTION. REFER TO THE APPROPRIATE


Fig 3

Fig.4


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6. The two rays of light reflected from the

mirror enter the eye as shown in Fig.5. Producing

the rays backward into the space behind the mirror is

shown by the dotted lines. These intersect at a point

P' which is as far behind the mirror as the red spot is

in front of the mirror. It is here that we say thatthere is an image of the red spot. The reason for this

is explained in Fig.6. which shows two light rays

coming from a real red spot located at this point in

the absence of the mirror.

As far as the eye is concerned it cannot

distinguish between the two situations shown in

Figures 5 and 6. The light rays entering the eye are

the same in both Figures. In Fig.5 the light appears

to come from the point P' and we say that an image

of the red spot is located there. This type of image,

where the light rays only appear to come from it, is

called a virtual image.

Fig.5

Fig6


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7.

In the situation described in the previous question you would know that it was only an image

that you were looking at because you would be aware of the mirror and know that the red spot

was really on your cheek.

Consider the situation in Fig.7. Your eye is looking into a box. One wall is a mirror and the

others are blackened. There is a lamp in a small alcove.

Draw in light rays from the filament of the lamp to the points A and B on the mirror

surface and their reflections into the eye.

Locate the image of the lamp.

Would you know that you were seeing a virtual image of the lamp rather than a real

lamp?



Fig.7


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7.

The light rays from the lamp and the reflected rays are shown in the Figure. Producing these

reflected rays backwards defines the location of the image of the lamp at L'. You see this image

at L' and are unable to distinguish this from a real lamp at this location.

You would be unable to detect the presence of the mirror since you could not see the reflection

of your eye or any features of the box since the walls are black.

Arrangements such as this form the basis of many illusions.

Fig8


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.

The figure shows two light rays which leave an object O. These strike a plane mirror,

are reflected and enter the eye as shown.

Draw in on the Figure the position of the mirror and accurately locate the position of the

image of O.



Fig.9


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8.

The position of the mirror is defined by the points at which the pairs of rays intersect

(You should also check that the law of reflection is obeyed here).

The position of the image is located by drawing backwards the rays A' and B'. As far as

the eye and the observer are concerned the rays A' and B' appear to come from this image point.

Fig10


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9 Let's imagine an ideal experiment. (Thought experiments are often useful in Physics.

Sometimes real-life situations are too complex to deal with.)

A fine beam of light is introduced through a small window, as shown in the Figure, into a box

which is evacuated (containing no air and hence no floating dust particles). All the walls are

perfectly black except for one which is a perfectly flat mirror. You look through another small

window in the wall opposite the mirror. (No light enters through this window-You are sitting in

a darkened room.)

Which of the following would best describe what you saw and why?

A. You would see nothing because light cannot travel in a vacuum.

B. You would see the box fully illuminated since it would be filled with light.

C. You would see the beam of light striking the mirror and being reflected by it.

D. You would see nothing because although light is present in the box none of it would reach

your eyes

ANS

Comment on the other answers.



Fig11


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9. D is correct. In this ideal situation the light after being reflected by the mirror will strike the

perfectly black wall and be completely absorbed there.

B is therefore incorrect. The box will not be filled with light . It will however contain

the beam of light striking the mirror and being reflected by it.

You would however not see these beams of light as suggested in Answer C. Remember

that you only see things when light enters your eye. You would see the beam from the head-

lamps of a car travelling on a distant road because some of the light is reflected sideways by

dust particles and this light reaches your eye. In our ideal box there is no air and no dust

particles and hence you do not see the beams. C is therefore incorrect.

A is incorrect. Light does travel through a vacuum for instance in reaching us from the

stars.

As explained above, D is correct. Light is present in the box as incident and reflectedbeams but is absorbed an striking the wall. None of this light reaches your eye.


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10.

You are looking into a small mirror as shown and see only a small part of your face

(from your forehead to your nose). You wish to see more of your face (e.g. you wish to see all

of your forehead and your mouth at the same time).

By drawing in the appropriate rays on the diagram decide whether you should:

A. move the mirror closer, for example to position A.

B. move the mirror further away for example to position B.

C. lower the mirror for example to position C.

or D. None of these will be effective.

ANS

Give the reason for your choice.



Fig 12


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11.

A cubical frame made of very fine wire rests as shown on a large flat mirror. Imagine that a

fine beam of light is sent from the corner of the cube marked O to strike the mirror at the pointC the centre of the square base of the cube.

The following questions will test your knowledge of the laws of reflection. You should indicate

in your answers whether the suggestions are allowed by these laws.

a) Could a reflected beam pass through the point L?

Yes/No

Give your reason.......................

.......................................

b) Could a reflected beam pass through the point D?

Yes/No

Give your reason.......................

.......................................

c) Could a reflected beam pass through the point M?

Yes/No

Give your reason.......................

.......................................

Now imagine a beam to be sent from O to strike the mirror at A the midpoint of the edge of the

cube.

d) Could a reflected beam pass through the point M?

Yes/NoGive your reason.......................

.......................................

e) Could a reflected beam pass through the point L?

Yes/No

Give your reason.......................

.......................................

f) Could a reflected beam pass through the point B (where LB = 1/4 LO)?

Yes/No

Give your reason.......................

Fig.14


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11. These questions test your knowledge of the laws of reflection.

I. The angle of incidence is equal to the angle of reflection i.e. i = r

II. The incident beam, the reflected beam and the normal (perpendicular) to the mirror must all

lie in one plane.

The answers are:

a) No (Law II is not satisfied).

b) No (Neither law is satisfied).

c) Yes (Both laws are satisfied).

d) No (Neither law is satisfied).

e) Yes (Both laws are satisfied).

f) No (Law I is not satisfied).

Fig.15


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Images and Plane Mirrors

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