ConcepTest 22.5ConcepTest 22.5 Heat InsulationHeat Insulation

Imagine you are an alien from another planet with Imagine you are an alien from another planet with infrared eyesinfrared eyes. .

What do you see when you look around the room?What do you see when you look around the room?

1) Bright spots where the bodies are and dark elsewhere.1) Bright spots where the bodies are and dark elsewhere.

2) Dark spots where the bodies are and bright elsewhere.2) Dark spots where the bodies are and bright elsewhere.

3) The same as what we see, only everything looks red.3) The same as what we see, only everything looks red.

4) The same as what we see, except that red is invisible.4) The same as what we see, except that red is invisible.

Bodies are sources of heat and therefore emit infrared radiationemit infrared radiation. An alien with an instrument to detect infrared would see these sources as bright spotssources as bright spots.

ConcepTest 22.5ConcepTest 22.5 Heat InsulationHeat Insulation

Imagine you are an alien from another planet with Imagine you are an alien from another planet with infrared eyesinfrared eyes. .

What do you see when you look around the room?What do you see when you look around the room?

1) Bright spots where the bodies are and dark elsewhere.1) Bright spots where the bodies are and dark elsewhere.

2) Dark spots where the bodies are and bright elsewhere.2) Dark spots where the bodies are and bright elsewhere.

3) The same as what we see, only everything looks red.3) The same as what we see, only everything looks red.

4) The same as what we see, except that red is invisible.4) The same as what we see, except that red is invisible.

Infrared photo of a building to check the heat insulation – where are the problem spots in this case?

ConcepTest 22.6ConcepTest 22.6 SupermanSuperman

1) Yes, no problem1) Yes, no problem

2) Nope, he can’t 2) Nope, he can’t

3) Need more information3) Need more information

Since Superman is from the planet Since Superman is from the planet

Krypton his eyes are sensitive to the Krypton his eyes are sensitive to the

entire electromagnetic spectrum. entire electromagnetic spectrum.

Does that mean he can use x-ray Does that mean he can use x-ray

vision to see that Lois Lane is being vision to see that Lois Lane is being

kidnapped in the other room?kidnapped in the other room?

X-ray vision means that Superman’s eyes

can receivereceive x-rays, but not sendsend them!

So what would have to happen for him to

see Lois Lane being kidnapped?

ConcepTest 22.6ConcepTest 22.6 SupermanSuperman

1) Yes, no problem1) Yes, no problem

2) Nope, he can’t 2) Nope, he can’t

3) Need more information3) Need more information

Since Superman is from the planet Since Superman is from the planet

Krypton his eyes are sensitive to the Krypton his eyes are sensitive to the

entire electromagnetic spectrum. entire electromagnetic spectrum.

Does that mean he can use x-ray Does that mean he can use x-ray

vision to see that Lois Lane is being vision to see that Lois Lane is being

kidnapped in the other room?kidnapped in the other room?

ConcepTest 23.2aConcepTest 23.2a Mirror IMirror I

S

O

1

2

3

4

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 23.2aConcepTest 23.2a Mirror IMirror I

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.

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 23.2bConcepTest 23.2b 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?

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

ConcepTest 23.2bConcepTest 23.2b 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?

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

ConcepTest 23.2cConcepTest 23.2c Mirror IIIMirror III

Does this depend on your Does this depend on your

distance from the mirror?distance from the mirror?

1) No.

2) Yes.

3) Depends on the mirror.

4) Depends on the person.

ConcepTest 23.2cConcepTest 23.2c Mirror IIIMirror III

Does this depend on your Does this depend on your

distance from the mirror?distance from the mirror?

1) No.

2) Yes.

3) Depends on the mirror.

4) 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 23.4aConcepTest 23.4a Refraction IRefraction I

1

air air

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

2

ConcepTest 23.4aConcepTest 23.4a Refraction IRefraction I

1

air air

The greater the

difference in the speed

of light between the two

media, the greater the

bending of the light

rays.

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

2

Follow-up:Follow-up: How does the speed in airair compare to that in 11 or 2?2?

ConcepTest 23.4bConcepTest 23.4b Refraction IIRefraction II

1

3

2

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) n1 > n2 > n3

2) n3 > n2 > n1

3) n2 > n3 > n1

4) n1 > n3 > n2

5) none of the above

ConcepTest 23.4bConcepTest 23.4b Refraction IIRefraction II

The rays are bent toward the normalbent toward the normal when crossing into #2, so nn22 > n > n11.

But rays are bent away from the bent away from the normalnormal when going into #3, so nn33 < n < n22.

How to find the relationship between #1 and #3? Ignore medium #2! So the rays are bent away from the normalbent away from the normal if they would pass from #1 directly into #3. Thus, we have: nn22 > n > n11 > n > n33 .

1

3

2

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) n1 > n2 > n3

2) n3 > n2 > n1

3) n2 > n3 > n1

4) n1 > n3 > n2

5) none of the above

ConcepTest 23.5aConcepTest 23.5a Gone Fishin’ IGone 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

ConcepTest 23.5aConcepTest 23.5a Gone Fishin’ IGone Fishin’ I

Due to refraction, the image will

appear higherhigher than the actual

fish, so you have to aimaim lowerlower to

compensate.

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

ConcepTest 23.5bConcepTest 23.5b Gone Fishin’ IIGone Fishin’ II

1) aim directly at the image

2) aim slightly above

3) aim slightly below

To shoot a fish with a laser gun,

should you aim directly at the

image, slightly above, or slightly

below?

The lightlight from the laser beam

will also bendbend when it hits the

air-water interface, so aimaim

directlydirectly at the fish at the fish.

ConcepTest 23.5bConcepTest 23.5b Gone Fishin’ IIGone Fishin’ II

laser beam

light from fish

1) aim directly at the image

2) aim slightly above

3) aim slightly below

To shoot a fish with a laser gun,

should you aim directly at the

image, slightly above, or slightly

below?

ConcepTest 24.1ConcepTest 24.1 SuperpositionSuperposition

1)

2)

3)

4)

If waves A and B are

superposed (that is, their

amplitudes are added) the

resultant wave is

The amplitudes of

waves A and B have to

be added at each

point!

ConcepTest 24.1ConcepTest 24.1 SuperpositionSuperposition

1)

2)

3)

4)

If waves A and B are

superposed (that is, their

amplitudes are added) the

resultant wave is

ConcepTest 24.2aConcepTest 24.2a Phase Difference IPhase 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

The two waves are out of phase by

1/4 wavelength1/4 wavelength (as seen in the

figure) , which corresponds to a

phase difference of 9090oo.

ConcepTest 24.2aConcepTest 24.2a Phase Difference IPhase Difference I1/4

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

Follow-up:Follow-up: What would the waves look like for no. 4 to be correct?

ConcepTest 24.2bConcepTest 24.2b Phase Difference IIPhase Difference II

1) out of phase by 180o

2) out of phase, but not by 180o

3) in phase

Two light sources emit waves of = 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

Since = 1 m, wave 1 has traveled twice this twice this

wavelengthwavelength while wave 2 has traveled three three

times this wavelengthtimes this wavelength. Thus, their phase

difference is one full wavelength,one full wavelength, which means

they are still in phase.

ConcepTest 24.2bConcepTest 24.2b Phase Difference IIPhase Difference II

1) out of phase by 180o

2) out of phase, but not by 180o

3) in phase

Two light sources emit waves of = 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

ConcepTest 24.3aConcepTest 24.3a Double Slits IDouble Slits I

1) spreads out

2) stays the same

3) shrinks together

4) disappears

In a double-slit experiment,

when the wavelength of the light

is increased, the interference

pattern

If is increased is increased and dd does does

not changenot change, then must must

increaseincrease, so the pattern

spreads out.

ConcepTest 24.3aConcepTest 24.3a Double Slits IDouble Slits I

1) spreads out

2) stays the same

3) shrinks together

4) disappears

dd sin sin = m = m

In a double-slit experiment,

when the wavelength of the light

is increased, the interference

pattern

ConcepTest 24.3bConcepTest 24.3b Double Slits IIDouble Slits II

1) spreads out

2) stays the same

3) shrinks together

4) disappears

If instead the If instead the slitsslits are moved are moved

farther apartfarther apart (without changing (without changing

the wavelength) the interference the wavelength) the interference

pattern pattern

If instead d is increased and does not change, then must decrease, so the pattern shrinks together

ConcepTest 24.3bConcepTest 24.3b Double Slits IIDouble Slits II

1) spreads out

2) stays the same

3) shrinks together

4) disappears

d sin = m

If instead the If instead the slitsslits are moved are moved

farther apartfarther apart (without changing (without changing

the wavelength) the interference the wavelength) the interference

pattern pattern

Follow-up:Follow-up: When would the interference pattern disappear?

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?

Inte

ns

ity

ConcepTest 24.4ConcepTest 24.4 Path DifferencePath Difference

Inte

ns

ity

7/

2

/2

3/

2

5/

2

For Destructive Interference

= 1/2 , 3/2 , 5/2 , 7/2 , …

= (m + 1/2)

23

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?

ConcepTest 24.4ConcepTest 24.4 Path DifferencePath Difference

ConcepTest 24.5aConcepTest 24.5a Diffraction IDiffraction 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

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.

ConcepTest 24.5aConcepTest 24.5a Diffraction IDiffraction I

dd

sin = d

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

ConcepTest 24.5bConcepTest 24.5b Diffraction IIDiffraction IIBlue light of wavelength 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 2, 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

ConcepTest 24.5bConcepTest 24.5b Diffraction IIDiffraction II

dd

d sin = m (minima)

If 2 2 then we must have dd

2d2d for sin for sin to remain unchanged to remain unchanged

(and thus give the same diffraction

pattern).

Blue light of wavelength 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 2, 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

ConcepTest 24.8ConcepTest 24.8 PolarizationPolarization

1) only case 1

2) only case 2

3) only case 3

4) cases 1 and 3

5) all three cases

If unpolarized light is incident

from the left, in which case will

some light get through?

ConcepTest 24.8 ConcepTest 24.8 PolarizationPolarization

In cases 1 and 3, light is

blocked by the adjacent

horizontal and vertical

polarizers. However, in case

2, the intermediate 45° intermediate 45°

polarizer allows some light polarizer allows some light

to get throughto get through the last

vertical polarizer.

1) only case 1

2) only case 2

3) only case 3

4) cases 1 and 3

5) all three cases

If unpolarized light is incident

from the left, in which case will

some light get through?