1. ConcepTest 22.5 Heat Insulation
Imagine you are an alien from another planet with infrared eyes. What do you see when you look around the room?
1) Bright spots where the bodies are and dark elsewhere.
2) Dark spots where the bodies are and bright elsewhere.
3) The same as what we see, only everything looks red.
4) The same as what we see, except that red is invisible.

2. ConcepTest 22.5 Heat Insulation
Imagine you are an alien from another planet with infrared eyes. What do you see when you look around the room?
1) Bright spots where the bodies are and dark elsewhere.
2) Dark spots where the bodies are and bright elsewhere.
3) The same as what we see, only everything looks red.
4) The same as what we see, except that red is invisible.
Bodies are sources of heat and therefore emit infrared radiation. An alien with an instrument to detect infrared would see these sources as bright spots.
Infrared photo of a building to check the heat insulation – where are the problem spots in this case?

3. ConcepTest 22.6 Superman
Since Superman is from the planet Krypton his eyes are sensitive to the entire electromagnetic spectrum. Does that mean he can use x-ray vision to see that Lois Lane is being kidnapped in the other room?
1) Yes, no problem
2) Nope, he can’t
3) Need more information

4. ConcepTest 22.6 Superman
Since Superman is from the planet Krypton his eyes are sensitive to the entire electromagnetic spectrum. Does that mean he can use x-ray vision to see that Lois Lane is being kidnapped in the other room?
1) Yes, no problem
2) Nope, he can’t
3) Need more information
X-ray vision means that Superman’s eyes can receive x-rays, but not send them! So what would have to happen for him to see Lois Lane being kidnapped?

5. ConcepTest 23.2a 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 1 2 3 4
mirror

6. ConcepTest 23.2a 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 1 2 3 4
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?

7. ConcepTest 23.2b Mirror II
1) same as your height
2) less than your full height but more than half your height
3) half your height
4) less than half your height
5) 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?

8. ConcepTest 23.2b Mirror II
1) same as your height
2) less than your full height but more than half your height
3) half your height
4) less than half your height
5) 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.

9. ConcepTest 23.2c Mirror III
1) No.
2) Yes.
3) Depends on the mirror.
4) Depends on the person.
Does this depend on your distance from the mirror?

10. ConcepTest 23.2c Mirror III
1) No.
2) Yes.
3) Depends on the mirror.
4) 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.

11. ConcepTest 23.4a Refraction I
Parallel light rays cross interfaces from air into two different media, 1 and 2, as shown in the figures below. In which of the media is the light traveling faster?
1) medium 1
2) medium 2
3) both the same 1
air
air 2

12. ConcepTest 23.4a Refraction I
Parallel light rays cross interfaces from air into two different media, 1 and 2, as shown in the figures below. In which of the media is the light traveling faster?
1) medium 1
2) medium 2
3) both the same
The greater the difference in the speed of light between the two media, the greater the bending of the light rays. 1
air
air 2
Follow-up: How does the speed in air compare to that in 1 or 2?

13. ConcepTest 23.4b Refraction II
1) n1 > n2 > n3
2) n3 > n2 > n1
3) n2 > n3 > n1
4) n1 > n3 > n2
5) none of the above
Parallel light rays cross interfaces from medium 1 into medium 2 and then into medium 3. What can we say about the relative sizes of the index of refraction of these media? 1 3 2

14. ConcepTest 23.4b Refraction II
1) n1 > n2 > n3
2) n3 > n2 > n1
3) n2 > n3 > n1
4) n1 > n3 > n2
5) none of the above
Parallel light rays cross interfaces from medium 1 into medium 2 and then into medium 3. What can we say about the relative sizes of the index of refraction of these media?
The rays are bent toward the normal when crossing into #2, so n2 > n1. But rays are bent away from the normal when going into #3, so n3 < n2. How to find the relationship between #1 and #3? Ignore medium #2! So the rays are bent away from the normal if they would pass from #1 directly into #3. Thus, we have: n2 > n1 > n3 . 1 3 2

15. ConcepTest 23.5a Gone Fishin’ I
To shoot a fish with a gun, should you aim directly at the image, slightly above, or slightly below?
1) aim directly at the image
2) aim slightly above
3) aim slightly below

16. ConcepTest 23.5a Gone Fishin’ I
To shoot a fish with a gun, should you aim directly at the image, slightly above, or slightly below?
1) aim directly at the image
2) aim slightly above
3) aim slightly below
Due to refraction, the image will appear higher than the actual fish, so you have to aim lower to compensate.

17. ConcepTest 23.5b Gone Fishin’ II
To shoot a fish with a laser gun, should you aim directly at the image, slightly above, or slightly below?
1) aim directly at the image
2) aim slightly above
3) aim slightly below

18. ConcepTest 23.5b Gone Fishin’ II
To shoot a fish with a laser gun, should you aim directly at the image, slightly above, or slightly below?
1) aim directly at the image
2) aim slightly above
3) aim slightly below
laser beam
light from fish
The light from the laser beam will also bend when it hits the air-water interface, so aim directly at the fish.

19. ConcepTest 24.1 SuperpositionA
If waves A and B are superposed (that is, their amplitudes are added) the resultant wave is B 1) 2) 3) 4)

20. ConcepTest 24.1 SuperpositionA
If waves A and B are superposed (that is, their amplitudes are added) the resultant wave is B 1) 2) 3) 4)
The amplitudes of waves A and B have to be added at each point!

21. ConcepTest 24.2a Phase Difference I
The two waves shown are
1) out of phase by 180o
2) out of phase by 90o
3) out of phase by 45o
4) out of phase by 360o
5) in phase
[CORRECT 5 ANSWER]

22. ConcepTest 24.2a Phase Difference I
1/4
The two waves shown are
1) out of phase by 180o
2) out of phase by 90o
3) out of phase by 45o
4) out of phase by 360o
5) in phase
The two waves are out of phase by 1/4 wavelength (as seen in the figure) , which corresponds to a phase difference of 90o.
Follow-up: What would the waves look like for no. 4 to be correct?

23. ConcepTest 24.2b Phase Difference II
Two light sources emit waves of l = 1 m which are in phase. The two waves from these sources meet at a distant point. Wave 1 traveled 2 m to reach the point, and wave 2 traveled 3 m. When the waves meet, they are
1) out of phase by 180o
2) out of phase, but not by 180o
3) in phase

24. ConcepTest 24.2b Phase Difference II
Two light sources emit waves of l = 1 m which are in phase. The two waves from these sources meet at a distant point. Wave 1 traveled 2 m to reach the point, and wave 2 traveled 3 m. When the waves meet, they are
1) out of phase by 180o
2) out of phase, but not by 180o
3) in phase
Since l = 1 m, wave 1 has traveled twice this wavelength while wave 2 has traveled three times this wavelength. Thus, their phase difference is one full wavelength, which means they are still in phase.

25. ConcepTest 24.3a Double Slits I
In a double-slit experiment, when the wavelength of the light is increased, the interference pattern
1) spreads out
2) stays the same
3) shrinks together
4) disappears

26. ConcepTest 24.3a Double Slits I
In a double-slit experiment, when the wavelength of the light is increased, the interference pattern
1) spreads out
2) stays the same
3) shrinks together
4) disappears
d sin  = m 
If  is increased and d does not change, then  must increase, so the pattern spreads out.

27. ConcepTest 24.3b Double Slits II
If instead the slits are moved farther apart (without changing the wavelength) the interference pattern
1) spreads out
2) stays the same
3) shrinks together
4) disappears

28. ConcepTest 24.3b Double Slits II
If instead the slits are moved farther apart (without changing the wavelength) the interference pattern
1) spreads out
2) stays the same
3) shrinks together
4) disappears
d sin  = m 
If instead d is increased and  does not change, then  must decrease, so the pattern shrinks together
Follow-up: When would the interference pattern disappear?

29. ConcepTest 24.4 Path Difference
1) there is no difference
2) half a wavelength
3) one wavelength
4) three wavelengths
5) more than three wavelengths
In a double-slit experiment, what path difference have the waves from each slit traveled to give a minimum at the indicated position?
Intensity

30. ConcepTest 24.4 Path Difference
1) there is no difference
2) half a wavelength
3) one wavelength
4) three wavelengths
5) more than three wavelengths
In a double-slit experiment, what path difference have the waves from each slit traveled to give a minimum at the indicated position?  2 3
For Destructive Interference
 = 1/2 , 3/2 , 5/2 , 7/2 , …
= (m + 1/2) 
Intensity
/2
3/2
5/2
7/2

31. ConcepTest 24.5a Diffraction I
The diffraction pattern below arises from a single slit. If we would like to sharpen the pattern, i.e., make the central bright spot narrower, what should we do to the slit width?
1) narrow the slit
2) widen the slit
3) enlarge the screen
4) close off the slit

32. ConcepTest 24.5a Diffraction I
The diffraction pattern below arises from a single slit. If we would like to sharpen the pattern, i.e., make the central bright spot narrower, what should we do to the slit width?
1) narrow the slit
2) widen the slit
3) enlarge the screen
4) close off the slit d q
The angle at which one finds the first minimum is:
The central bright spot can be narrowed by having a smaller angle. This in turn is accomplished by widening the slit.
sin  =  / d

33. ConcepTest 24.5b Diffraction II
Blue light of wavelength l passes through a single slit of width d and forms a diffraction pattern on a screen. If the blue light is replaced by red light of wavelength 2l, the original diffraction pattern can be reproduced if the slit width is changed to:
1) d/4
2) d/2
3) no change needed
4) 2 d
5) 4 d

34. ConcepTest 24.5b Diffraction II
Blue light of wavelength l passes through a single slit of width d and forms a diffraction pattern on a screen. If the blue light is replaced by red light of wavelength 2l, the original diffraction pattern can be reproduced if the slit width is changed to:
1) d/4
2) d/2
3) no change needed
4) 2 d
5) 4 d d q
d sin = m (minima)
If   2 then we must have d  2d for sin  to remain unchanged (and thus give the same diffraction pattern).

35. ConcepTest 24.8 Polarization
If unpolarized light is incident from the left, in which case will some light get through?
1) only case 1
2) only case 2
3) only case 3
4) cases 1 and 3
5) all three cases 1 2 3

36. ConcepTest 24.8 Polarization
If unpolarized light is incident from the left, in which case will some light get through?
1) only case 1
2) only case 2
3) only case 3
4) cases 1 and 3
5) all three cases 1 2 3
In cases 1 and 3, light is blocked by the adjacent horizontal and vertical polarizers. However, in case 2, the intermediate 45° polarizer allows some light to get through the last vertical polarizer.