ConcepTest 7.2aConcepTest 7.2a Mirror IMirror I

S

O

a

b

c

d

mirrorAn observer at An observer at point Opoint O is facing a is facing a

mirror and observes a mirror and observes a light source light source

SS. Where does the observer . Where does the observer

perceive the mirror image of the perceive the mirror image of the

source to be located?source to be located?

ConcepTest 7.2aConcepTest 7.2a Mirror IMirror I

S

O

a

b

c

d

mirror

Trace the light rays from the object

to the mirror to the eye. Since the

brain assumes that light travels in a

straight line, simply extend the rays

back behind the mirror to locate the

image.

An observer at An observer at point Opoint O is facing a is facing a

mirror and observes a mirror and observes a light source light source

SS. Where does the observer . Where does the observer

perceive the mirror image of the perceive the mirror image of the

source to be located?source to be located?

Follow-up:Follow-up: What happens when the observer starts moving What happens when the observer starts moving toward the mirror?toward the mirror?

ConcepTest 7.2bConcepTest 7.2b Mirror IIMirror II

You stand in front of a You stand in front of a

mirror. How tall does the mirror. How tall does the

mirror have to be so that mirror have to be so that

you can see yourself you can see yourself

entirely?entirely?

a) same as your height

b) less than your full height but more than half your height

c) half your height

d) less than half your height

e) any size will do

ConcepTest 7.2bConcepTest 7.2b Mirror IIMirror II

Trace the light rays from the

image’s foot to the mirror and

then to the eye. Since we know

that ii = = rr , you need a mirror mirror

only half your sizeonly half your size.

You stand in front of a You stand in front of a

mirror. How tall does the mirror. How tall does the

mirror have to be so that mirror have to be so that

you can see yourself you can see yourself

entirely?entirely?

a) same as your height

b) less than your full height but more than half your height

c) half your height

d) less than half your height

e) any size will do

ConcepTest 7.2cConcepTest 7.2c Mirror IIIMirror III

Does this depend on your Does this depend on your

distance from the mirror?distance from the mirror?

a) No.

b) Yes.

c) Depends on the mirror.

d) Depends on the person.

ConcepTest 7.2cConcepTest 7.2c Mirror IIIMirror III

Does this depend on your Does this depend on your

distance from the mirror?distance from the mirror?

a) No.

b) Yes.

c) Depends on the mirror.

d) Depends on the person.

The further you step back, the

smaller the incident and

reflected angles will be. But the

rays will still be reflected at the

same points, so the ray from

the foot will still be reflected at

mid-height.

ConcepTest 7.3ConcepTest 7.3 All Smoke and MirrorsAll Smoke and Mirrors

1.0 m 0.5 m

a) 0.5 m

b) 1.0 m

c) 1.5 m

d) 2.0 m

e) 2.5 m

You hold a hand mirror You hold a hand mirror 0.5 m0.5 m in in

front of you and look at your front of you and look at your

reflection in a full-length mirror reflection in a full-length mirror 1 m1 m

behind you. behind you. How far in back of How far in back of

the big mirror do you see the image the big mirror do you see the image

of your face?of your face?

The image of the face reflected in the

small mirrorsmall mirror appears 0.5 m0.5 m behind the

small mirror. This image (which is

the object for the big mirrorbig mirror) is 2.0 m2.0 m

away from the big mirror. The final

image is 2.0 m2.0 m behind the big mirrorbig mirror.

ConcepTest 7.3ConcepTest 7.3 All Smoke and MirrorsAll Smoke and Mirrors

1.0 m 0.5 m

a) 0.5 m

b) 1.0 m

c) 1.5 m

d) 2.0 m

e) 2.5 m

You hold a hand mirror You hold a hand mirror 0.5 m0.5 m in in

front of you and look at your front of you and look at your

reflection in a full-length mirror reflection in a full-length mirror 1 m1 m

behind you. behind you. How far in back of How far in back of

the big mirror do you see the image the big mirror do you see the image

of your face?of your face?

ConcepTest 7.4ConcepTest 7.4 Fire and MirrorsFire and Mirrors

a) Flat Mirror

b) Concave Mirror

c) Convex Mirror

You want to set fire to a pile of dry You want to set fire to a pile of dry

leaves. Which of these mirrors is leaves. Which of these mirrors is

the best tool:the best tool:

ConcepTest 7.4ConcepTest 7.4 Fire and MirrorsFire and Mirrors

a) Flat Mirror

b) Concave Mirror

c) Convex Mirror

You want to set fire to a pile of dry You want to set fire to a pile of dry

leaves. Which of these mirrors is leaves. Which of these mirrors is

the best tool:the best tool:

The concave mirror, or converging

mirror is the only one that will work.

This mirror focuses light from the

Sun into a small point and causes the

leaves to heat up.

Follow-up:Follow-up: What type of lens is best for this purpose? What type of lens is best for this purpose?

ConcepTest 7.5ConcepTest 7.5 What mirror is this?What mirror is this?

a) Concave, real

b) Concave, virtual

c) Convex, real

d) Convex, virtual

e) Flat, virtual

These folks are looking into a These folks are looking into a

mirror. Based on the image, which mirror. Based on the image, which

of these is true about the mirror of these is true about the mirror

and image?and image?

a) Concave, real

b) Concave, virtual

c) Convex, real

d) Convex, virtual

e) Flat, virtual

These folks are looking into a These folks are looking into a

mirror. Based on the image, which mirror. Based on the image, which

of these is true about the mirror of these is true about the mirror

and image?and image?

Because the image is bigger than the

object, the mirror can be neither flat

or convex. For flat mirrors, M=1, and,

for convex, M<1. Therefore, the

mirror must be concave. The image

is virtual because it is upright (so

q<0).

ConcepTest 7.5ConcepTest 7.5 What mirror is this?What mirror is this?

ConcepTest 7.6ConcepTest 7.6 GlassGlass

a) Zero

b) Infinity

c) Thickness of the glass

d) Impossible to determine

What is the focal length of a pane What is the focal length of a pane

of window glass?of window glass?

ConcepTest 7.6ConcepTest 7.6 GlassGlass

a) Zero

b) Infinity

c) Thickness of the glass

d) Impossible to determine

What is the focal length of a pane What is the focal length of a pane

of window glass?of window glass?

The focal length of the glass is

infinity. The focal length of a lens is

½ of its radius. The flat glass is like a

really big sphere with a radius of

infinity.

ConcepTest 7.7ConcepTest 7.7 AberrationsAberrations

a) spherical aberration b) chromatic aberration c) both spherical

aberration and chromatic aberration

A curved mirror surface can have:A curved mirror surface can have:

ConcepTest 7.7ConcepTest 7.7 AberrationsAberrations

a) spherical aberration b) chromatic aberration c) both spherical

aberration and chromatic aberration

A curved mirror surface can have:A curved mirror surface can have:

Curved mirrors do not exhibit

chromatic aberration because the light

is not traveling through the glass

medium. Chromatic aberration is due

to the wavelength dependence of the

index of refraction.

ConcepTest 7.8ConcepTest 7.8 CamerasCameras

a) Moved away from the film

b) Moved toward the film

c) Left where it is.

A camera can be modeled as a simple

converging lens that focuses an image

on the film, acting as the screen. A

camera is initially focused on a distant

object. To focus the image of an object

close to the camera, the lens must be

_____.

ConcepTest 7.8ConcepTest 7.8 CamerasCameras

a) Moved away from the film

b) Moved toward the film

c) Left where it is.

A camera can be modeled as a simple

converging lens that focuses an image

on the film, acting as the screen. A

camera is initially focused on a distant

object. To focus the image of an object

close to the camera, the lens must be

_____.

When the object is far away, the image is at the focal point.

When the object is brought closer to the lens (but outside f),

the image is behind the focal point.

ConcepTest 7.9ConcepTest 7.9 Age Age

a) 10-12 seconds older

b) 10-9 seconds older

c) Same age

d) 10-12 seconds younger

e) 10-9 seconds younger

Does your bathroom mirror show you

older or younger than you actually

are? Compute an order-of-magnitude

estimate for the age difference, based

on data that you specify.

ConcepTest 7.9ConcepTest 7.9 Mirror of YouthMirror of Youth

a) 10-12 seconds older

b) 10-9 seconds older

c) Same age

d) 10-12 seconds younger

e) 10-9 seconds younger

Does your bathroom mirror show you

older or younger than you actually

are? Compute an order-of-magnitude

estimate for the age difference, based

on data that you specify.

Assume the mirror is 1 meter

away from you. This image that

you see in the mirror is from light

that left your body some time ago.

That time is t=d/v=2m/3x108 m/s ~

10-9 s