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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