MT 2

Question 1

Why do radio waves diffract around buildings, while light waves do not?

A. Radio waves travel much slower than light waves
B. Radio waves are electromagnetic waves while light waves are not.
C. Radio waves have a much longer wavelengths than light waves.

Answer 1

The wavelengths of AM radio waves are hundreds of meters, much larger than the size of buildings, so they easily diffract around buildings. Light, with wavelengths a tiny fraction of a centimeter, show no appreciable diffraction around buildings. Answer C.

Question 2

Which waves will likely experience interference when they pass between atomic planes in solids?

A. Radio waves
B. Lightwaves
C. X-rays

Answer 2

These must be X-rays because their wavelength is comparable with the distance between the atomic planes. Answer C.

Question 3

Diffraction gratings are used to determine

A. The velocity of light in the medium
B. The spectrum of light wavelengths emitted by a source
C. The distance between atoms in solids

Answer 3

B. The spectrum of light wavelengths emitted by a source

Question 4

When two ideal gases are mixed and reach thermal equilibrium

A. They have the same temperatures
B. They occupy the same volume
C. They have the same average kinetic energy 
D. All of the above
E. None of the above

Answer 4

D. All of the above

Question 5

Interference effects are produced at arbitrary point P on a very distant screen as a result of direct rays from a source S and reflected rays off the mirror, as shown. Exactly above the point O when the screen touches the mirror, the interference picture will be

A. At its maximum
B. At its minimum
C. Neither maximum nor minimum

Answer 5

Since the mirror has a higher refraction index than the air, the reflected light undergoes the phase change by half cycle therefore the conditions of the constructive and destructive interference interchange. That is dsinθ=mλ is the condition for destructive interference.
dsinθ=(m+1/2)λ is the condition for constructive interference.
In particular we see that the angle θ=0 (plane of the mirror) corresponds to the dark fringe. Answer B.

Question 6

X-ray diffraction is used to determine

A. Velocity of light in the medium
B. Distances between atoms in solids
C. Spectrum of light wavelengths emitted by a source D. Refractive index of the medium
E. All of the above

Answer 6

Solution. X-ray diffraction is used to determine distances between atoms in solids. Answer B.

Question 7

You turn on the faucet and observe a steady stream of water as shown to the right. You notice that the water stream gets narrower as it falls. Use your knowledge of fluid flow to explain why the water stream gets narrower. (Eventually, the stream breaks up into small droplets—you do not need to explain this. You may also ignore any effects of air friction, turbulence, and surface tension).

Answer 7

Since the water is in the air as it falls the pressure on it during its fall is a constant 1atm. Bernoulli’s eq. only has changes in K/volume and Ugrav/volume so as Ugrav decreases K must increase which means that the speed, v, must increase. By our continuity equation, A1v1 = A2v2, we see that larger v means smaller cross- sectional area, A, so the stream must get narrower.

Question 8

If you drive up the mountain with a bag of potato chips, you will find it inflated (or even burst open) because

1. The temperature on top of the mountain is smaller than the temperature of the chips
2. The temperature on top of the mountain is larger than the temperature of the chips
3. The pressure on top of the mountain is smaller than the pressure inside the bag
4. The pressure on top of the mountain is larger than the pressure inside the bag

Answer 8

The pressure on top of the mountain is smaller than the pressure inside the bag

Question 9

If you measure car tire pressure before starting to drive and after a long drive

1. The tire pressure is larger after long drive because the tire gets hot.
2. The tire pressure is smaller after long drive because some air leaks out.
3. The tire pressure remains the same regardless if you have driven the car or not

Answer 9

The tire pressure is larger after long drive because the tire gets hot.

Question 10

A gas of particles is described by the van der Waals equation of state. Under which conditions do the van der Waals corrections become insignificant, i.e. the gas can be described by the ideal gas equation of state?

1. In the limit of high temperatures
2. In the limit of small temperatures
3. In the limit of high densities
4. In the limit of low densities

Answer 10

4. In the limit of low densities

Question 11

The Kelvin temperature scale is

1. The same as the Fahrenheit scale with zero shifted to -273oF.
2. The same as the Celsius scale with zero shifted to -273oC.
3. Taken as the average of the Celsius and Fahrenheit scales.

Answer 11

2.The same as the Celsius scale with zero shifted to -273oC.

Question 12

Which statement is correct if any

1. Two bodies in thermal equilibrium have the same temperatures
2. Two bodies in thermal equilibrium have the same amount of heat
3. Both statements are right.
4. Both statements are wrong.

Answer 12

1.Two bodies in thermal equilibrium have the same temperatures

Question 13

If most materials shrink with decreasing temperature why upon freezing water pipes can blow up

1. It is connected to the positive thermal expansion coefficient of water around 0oC.
2. It is connected to the zero thermal expansion coefficient of water around 0oC.
3. It is connected to the negative thermal expansion coefficient of water around 0oC.

Answer 13

3.It is connected to the negative thermal expansion coefficient of water around 0oC.

Question 14

When an object with a hole undergoes thermal expansion, the hole

Answer 14

will expand

Question 15

To cool 1 liter of water from 10oC to 0oC you need to take out an amount of heat equal to Q=mcΔT=1419010 =41900 Joules. How much heat do you need to take out in order to transform the same water into ice at 0oC.

1. 41900 Joules
2. More than 41900 Joules
3. Less than 41900 Joules

Answer 15

More than 41900 Joules

Question 16

What is sublimation?

1. Transformation from solid phase of matter to its liquid phase
2. Transformation from liquid phase of matter to its gaseous phase
3. Transformation from solid phase of matter to its gaseous phase by skipping the liquid (is it possible?)

Answer 16

Transformation from solid phase of matter to its gaseous phase by skipping the liquid (is it possible?)

Question 17

What are the mechanisms of the heat transfer?

1. Conduction
2. Conduction, Convection and Radiation
3. Conduction, Vaporization and Combustion

Answer 17

Conduction, Convection and Radiation

Question 18

A laser beam is directed onto a wide slit and the diffraction picture is seen on the screen behind it. The diffraction minima occur when the path difference between the waves from the slit is equal to

1. Integer number of wavelengths
2. Odd integer number of wavelengths
3. Odd integer number of half-wavelengths
4. There is no simple relationship

Answer 18

1.Integer number of wavelengths

Question 19

What is the difference between the Fraunhofer and Fresnel diffraction

1. Fraunhofer diffraction considers rays that are approximately parallel (distant screen limit), Fresnel diffraction considers rays that are not parallel (the screen is close)
2. Fraunhofer diffraction considers rays that are not parallel (the screen is close) , Fresnel diffraction considers rays that are approximately parallel (distant screen) 3.Fraunhofer diffraction considers wide slits, Fresnel diffraction considers narrow slits
3. Fraunhofer diffraction considers narrow slits, Fresnel diffraction considers wide slits

Answer 19

1.Fraunhofer diffraction considers rays that are approximately parallel (distant screen limit), Fresnel diffraction considers rays that are not parallel (the screen is close)

Question 20

Diffraction gratings are used to determine

1. The velocity of light in the medium
2. The spectrum of light wavelengths emitted by a source
3. The distance between atoms in solids

Answer 20

2.The spectrum of light wavelengths emitted by a source

Question 21

What is the typical wavelength of X-rays?

1. a meter
2. a millimeter
3. a micrometer
4. 10-10 m (Angstrom)

Answer 21

10-10 m (Angstrom)

Question 22

A laser beam is directed onto two slits and the interference picture is seen on the screen behind it. The interference maxima occur when the path difference between the waves from each slit is equal to

1. Integer number of wavelengths
2. Odd integer number of wavelengths
3. Odd integer number of half-wavelengths

Answer 22

1.Integer number of wavelengths

Question 23

If the path difference between two waves coming to a particular point is 3 wavelengths, the corresponding phase difference is

1. 3π/4
2. 3π/2
3. 3 π
4. 4 π
5. 6 π

Answer 23

6 π

Question 24

If the path difference between two waves coming to a particular point is 3/2 wavelengths, the corresponding phase difference is

1. 3π/4
2. 3π/2
3. 3 π
4. 4 π
5. 6 π

Answer 24

3 π

Question 25

If light is reflected from a window, the reflected light

1. has no phase shift with respect to incident light
2. has a phase shift equal to π/2 with respect incident light 3.has a phase shift equal to π with respect incident light 4.has a phase shift equal to 3π/2 with respect incident light

Answer 25

3.has a phase shift equal to π with respect incident light

Question 26

If light is passing through a window, the transmitted light

1. has no phase shift with respect to incident light
2. has a phase shift equal to π/2 with respect incident light 3.has a phase shift equal to π with respect incident light 4.has a phase shift equal to 3π/2 with respect incident light

Answer 26

has no phase shift with respect to incident light

Question 27

The rainbow fringes seen on an oil drop in a glass of water are due to

1. Interference between rays reflected from the top and bottom surfaces of the oil drop
2. Interference between rays reflected from the top surface of the oil drop and the bottom of the glass
3. Interference between rays reflected from the bottom surface of the oil drop and the bottom of the glass

Answer 27

Interference between rays reflected from the top and bottom surfaces of the oil drop

Question 28

If light is reflected from a window, the reflected light

A. has no phase shift with respect to incident light
B. has a phase shift equal to π/2with respect incident light C. has a phase shift equal to π with respect incident light
D. has a phase shift equal to 3π/2 with respect incident light

Answer 28

Since window glass has its refraction index larger than the refraction index of the air, the light will reflect with phase shift equal to π. Answer C.

Question 29

Coherent electromagnetic radiation is sent through a slit of width 0.01 mm. For which of the following wavelengths will there be no points in the diffraction pattern where the intensity is zero?

A. Blue light of wavelength 500 nm
B. Infrared light of wavelength 10.6 μm
C. Ultraviolet light of wavelength 50 nm.

Answer 29

Solution. If the slit width a is less than the wavelength, there are no points at the diffraction pattern at which the intensity is zero. Answer B.

Question 30

If the path difference between two waves coming to a particular point is 3/8 wavelengths, the corresponding phase difference is

Α. 3π/8
B. 3π/4
C. 3 π/2
D. 2π
E. 3π

Answer 30

Solution. One wavelength corresponds to the phase difference of 2π. Consequently 3/8 of the wavelength corresponds to 2π*3/8= 3π/4. Answer B.

Question 31

If the path difference between two waves coming to a particular point is 3/2 wavelengths, the corresponding phase difference is

Α. 3π/8
B. 3π/4
C. 3 π/2
D. 2π
E. 3π

Answer 31

Solution. One wavelength corresponds to the phase difference of 2π. Consequently 3/2 of the wavelength corresponds to 2π*3/2= 3π. Answer E.

Question 32

Which waves will likely experience interference when they pass between two skyscrapers?

A. Radio waves
B. Lightwaves
C. X-rays

Answer 32

Solution. These must be radio waves because their wavelength is comparable with the distance between the buildings. Answer A.

Question 33

If most materials shrink with decreasing temperature why upon freezing water pipes can blow up
A. It is connected to the positive thermal expansion coefficient of water around 0oC.
B. It is connected to the zero thermal expansion coefficient of water around 0oC.
C. It is connected to the negative thermal expansion coefficient of water around 0oC.

Answer 33

Solution. Answer C.

Question 34

At a certain depth of the lake the absolute pressure is p. At twice this depth will the absolute pressure be equal to

A. 2p
B. larger than2p
C. less than 2p
D. still equal p

Answer 34

Solution. The pressure inside the lake at depth h is given by p(h)=p0+ρgh therefore the ratio p(2h)/p(h)=(p0+2ρgh)/(p0+ρgh) is always less than 2 as long as p0 (atmospheric pressure) is larger than zero. Answer C.