1. ConcepTest 7.2a Mirror I
An observer at point O is facing a mirror and observes a light source S. Where does the observer perceive the mirror image of the source to be located? S O a b c d
mirror

2. ConcepTest 7.2a Mirror I
An observer at point O is facing a mirror and observes a light source S. Where does the observer perceive the mirror image of the source to be located? 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.
Follow-up: What happens when the observer starts moving
toward the mirror?

3. ConcepTest 7.2b Mirror II
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
You stand in front of a mirror. How tall does the mirror have to be so that you can see yourself entirely?

4. ConcepTest 7.2b Mirror II
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
You stand in front of a mirror. How tall does the mirror have to be so that you can see yourself entirely?
Trace the light rays from the image’s foot to the mirror and then to the eye. Since we know that qi = qr , you need a mirror only half your size.

5. ConcepTest 7.2c Mirror III
a) No.
b) Yes.
c) Depends on the mirror.
d) Depends on the person.
Does this depend on your distance from the mirror?

6. ConcepTest 7.2c Mirror III
a) No.
b) Yes.
c) Depends on the mirror.
d) Depends on the person.
Does this depend on your distance from the mirror?
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.

7. ConcepTest 7.3 All Smoke and Mirrors
You hold a hand mirror 0.5 m in front of you and look at your reflection in a full-length mirror 1 m behind you. How far in back of the big mirror do you see the image of your face?
a) 0.5 m
b) 1.0 m
c) 1.5 m
d) 2.0 m
e) 2.5 m
1.0 m
0.5 m

8. ConcepTest 7.3 All Smoke and Mirrors
You hold a hand mirror 0.5 m in front of you and look at your reflection in a full-length mirror 1 m behind you. How far in back of the big mirror do you see the image of your face?
a) 0.5 m
b) 1.0 m
c) 1.5 m
d) 2.0 m
e) 2.5 m
1.0 m
0.5 m
The image of the face reflected in the small mirror appears 0.5 m behind the small mirror. This image (which is the object for the big mirror) is 2.0 m away from the big mirror. The final image is 2.0 m behind the big mirror.

9. ConcepTest 7.4 Fire and Mirrors
You want to set fire to a pile of dry leaves. Which of these mirrors is the best tool:
a) Flat Mirror
b) Concave Mirror
c) Convex Mirror

10. ConcepTest 7.4 Fire and Mirrors
You want to set fire to a pile of dry leaves. Which of these mirrors is the best tool:
a) Flat Mirror
b) Concave Mirror
c) Convex Mirror
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: What type of lens is best for this purpose?

11. ConcepTest 7.5 What mirror is this?
These folks are looking into a mirror. Based on the image, which of these is true about the mirror and image?
a) Concave, real
b) Concave, virtual
c) Convex, real
d) Convex, virtual
e) Flat, virtual

12. ConcepTest 7.5 What mirror is this?
These folks are looking into a mirror. Based on the image, which of these is true about the mirror and image?
a) Concave, real
b) Concave, virtual
c) Convex, real
d) Convex, virtual
e) Flat, virtual
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).

13. ConcepTest 7.6 Glass
What is the focal length of a pane of window glass?
a) Zero
b) Infinity
c) Thickness of the glass
d) Impossible to determine

14. ConcepTest 7.6 Glass
What is the focal length of a pane of window glass?
a) Zero
b) Infinity
c) Thickness of the glass
d) Impossible to determine
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.

15. ConcepTest 7.7 Aberrations
A curved mirror surface can have:
a) spherical aberration
b) chromatic aberration
c) both spherical aberration and chromatic aberration

16. ConcepTest 7.7 Aberrations
A curved mirror surface can have:
a) spherical aberration
b) chromatic aberration
c) both spherical aberration and chromatic aberration
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.

17. ConcepTest 7.8 Cameras
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 _____.
a) Moved away from the film
b) Moved toward the film
c) Left where it is.

18. ConcepTest 7.8 Cameras
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 _____.
a) Moved away from the film
b) Moved toward the film
c) Left where it is.
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.

19. ConcepTest 7.9 Age b) 10-9 seconds older c) Same age
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.
a) seconds older
b) 10-9 seconds older
c) Same age
d) seconds younger
e) 10-9 seconds younger

20. ConcepTest 7.9 Mirror of Youth
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.
a) seconds older
b) 10-9 seconds older
c) Same age
d) seconds younger
e) 10-9 seconds younger
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