Chapter Review
Concept Items
21.1 Planck and Quantum Nature of Light
What aspect of the blackbody spectrum forced Planck to propose quantization of energy levels in atoms and molecules?
- Radiation occurs at a particular frequency that does not change with the energy supplied.
- Certain radiation occurs at a particular frequency that changes with the energy supplied.
- Maximum radiation would occur at a particular frequency that does not change with the energy supplied.
- Maximum radiation would occur at a particular frequency that changes with the energy supplied.
Two lasers shine red light at 650 nm. One laser is twice as bright as the other. Explain this difference using photons and photon energy.
- The brighter laser emits twice the number of photons and more energy per photon.
- The brighter laser emits twice the number of photons and less energy per photon.
- Both lasers emit equal numbers of photons and equivalent amounts of energy per photon.
- The brighter laser emits twice the number of photons but both lasers emit equivalent amounts of energy per photon.
Consider four stars in the night sky: red, yellow, orange, and blue. The photons of which star will carry the greatest amount of energy?
- blue
- orange
- red
- yellow
- The bulb will emit greener light.
- The bulb will emit bluer light.
- The bulb will emit more ultraviolet light.
- The bulb will emit redder light.
21.2 Einstein and the Photoelectric Effect
- An increase in light intensity decreases the number of photons. However, no electrons are ejected.
- Increase in light intensity increases the number of photons, so electrons with higher kinetic energy are ejected.
- An increase in light intensity increases the number of photons, so electrons will be ejected.
- An increase in light intensity increases the number of photons. However, no electrons are ejected.
True or false—The concept of a work function (or binding energy) is permissible under the classical wave model.
- false
- true
- No, there is not enough energy associated with a single microwave photon to result in cell damage.
- No, there is zero energy associated with a single microwave photon, so it does not result in cell damage.
- Yes, a single microwave photon causes cell damage because it does not have high energy.
- Yes, a single microwave photon causes cell damage because it has enough energy.
21.3 The Dual Nature of Light
- The momentum of a singular photon is incredibly small.
- The momentum is not felt because very few photons strike us at any time, and not all have momentum.
- The momentum of a singular photon is large, but very few photons strike us at any time.
- A large number of photons strike us at any time, and so their combined momentum is incredibly large.
- No, an interference pattern will not emerge because helium atoms will strike a variety of locations on the screen.
- No, an interference pattern will not emerge because helium atoms will strike at certain locations on the screen.
- Yes, an interference pattern will emerge because helium atoms will strike a variety of locations on the screen.
- Yes, an interference pattern will emerge because helium atoms will strike at certain locations on the screen.
Critical Thinking Items
21.1 Planck and Quantum Nature of Light
- Frequency is inversely proportional to temperature.
- Frequency is directly proportional to temperature.
- Frequency is directly proportional to the square of temperature.
- Frequency is directly proportional to the fourth power of temperature.
- Because after reaching the peak frequency, the photons created at a particular frequency are too many for energy intensity to continue to decrease.
- Because after reaching the peak frequency, the photons created at a particular frequency are too few for energy intensity to continue to decrease.
- Because after reaching the peak frequency, the photons created at a particular frequency are too many for energy intensity to continue to increase.
- Because after reaching the peak frequency, the photons created at a particular frequency are too few for energy intensity to continue to increase.
- Blue-violet light contains greater amount of energy than red light.
- Blue-violet light contains lower amount of energy than red light.
- Both blue-violet light and red light have the same frequency but contain different amounts of energy.
- Blue-violet light frequency is lower than the frequency of red light.
Why is it assumed that a perfect absorber of light (like a blackbody) must also be a perfect emitter of light?
- To achieve electrostatic equilibrium with its surroundings
- To achieve thermal equilibrium with its surroundings
- To achieve mechanical equilibrium with its surroundings
- To achieve chemical equilibrium with its surroundings
21.2 Einstein and the Photoelectric Effect
- As frequency is increased, electrons will stop being ejected from the surface.
- As frequency is increased, electrons will begin to be ejected from the surface.
- As frequency is increased, it will have no effect on the electrons being ejected as the intensity is the same.
- As frequency is increased, the rate at which the electrons are being ejected will increase.
Why is it important to consider what material to use when designing a light meter? Consider the worked example from Section 21-2 for assistance.
- A light meter should contain material that responds only to high frequency light.
- A light meter should contain material that responds to low frequency light.
- A light meter should contain material that has high binding energy.
- A light meter should contain a material that does not show any photoelectric effect.
Why does overexposure to UV light often result in sunburn when overexposure to visible light does not? This is why you can get burnt even on a cloudy day.
- UV light carries less energy than visible light and can penetrate our body.
- UV light carries more energy than visible light, so it cannot break bonds at the cellular level.
- UV light carries more energy than visible light and can break bonds at the cellular level.
- UV light carries less energy than visible light and cannot penetrate the human body.
If you pick up and shake a piece of metal that has electrons in it free to move as a current, no electrons fall out. Yet if you heat the metal, electrons can be boiled off. Explain both of these facts as they relate to the amount and distribution of energy involved with shaking the object as compared with heating it.
- Thermal energy is added to the metal at a much higher rate than energy added due to shaking.
- Thermal energy is added to the metal at a much lower rate than energy added due to shaking.
- If the thermal energy added is below the binding energy of the electrons, they may be boiled off.
- If the mechanical energy added is below the binding energy of the electrons, they may be boiled off.
21.3 The Dual Nature of Light
- Because, photons and electrons do not exist on the molecular level, all energy of motion is considered kinetic energy.
- Because, photons exist on the molecular level while electrons do not exist on the molecular level, all energy of motion is considered kinetic energy.
- Because, electrons exist on the molecular level while photons do not exist on the molecular level, all energy of motion is considered kinetic energy.
- Because, photons and electrons exist on the molecular level, all energy of motion is considered kinetic energy.
In what region of the electromagnetic spectrum will photons be most effective in accelerating a solar sail?
- ultraviolet rays
- infrared rays
- X-rays
- gamma rays
True or false—Electron microscopes can resolve images that are smaller than the images resolved by light microscopes.
- false
- true
- Ultraviolet light carries less energy than X-rays. As a result, Compton scattering would be easier to detect.
- Ultraviolet light carries less energy than X-rays. As a result, Compton scattering would be more difficult to detect.
- Ultraviolet light carries more energy than X-rays. As a result, Compton scattering would be easier to detect.
- Ultraviolet light has higher energy than X-rays. As a result, Compton scattering would be more difficult to detect.
Problems
21.1 Planck and Quantum Nature of Light
How many X-ray photons per second are created by an X-ray tube that produces a flux of X-rays having a power of 1.00 W? Assume the average energy per photon is 75.0 keV.
- 8.33 × 1015 photons
- 9.1 × 107 photons
- 9.1 × 108 photons
- 8.33 × 1013 photons
What is the frequency of a photon produced in a CRT using a 25.0-kV accelerating potential? This is similar to the layout as in older color television sets.
- 6.04 × 10−48 Hz
- 2.77 × 10−48 Hz
- 3.02 × 1018 Hz
- 6.04 × 1018 Hz
21.2 Einstein and the Photoelectric Effect
What is the binding energy in eV of electrons in magnesium, if the longest-wavelength photon that can eject electrons is 337 nm?
- 7.44 × 10−19 J
- 7.44 × 10−49 J
- 5.90 × 10−17 J
- 5.90 × 10−19 J
Photoelectrons from a material with a binding energy of 2.71 eV are ejected by 420-nm photons. Once ejected, how long does it take these electrons to travel 2.50 cm to a detection device?
- 8.5 × 10−6 s
- 3.5 × 10−7 s
- 43.5 × 10−9 s
- 8.5 × 10−8 s
21.3 The Dual Nature of Light
What is the momentum of a 0.0100-nm-wavelength photon that could detect details of an atom?
- 6.626 × 10−27 kg ⋅ m/s
- 6.626 × 10−32 kg ⋅ m/s
- 6.626 × 10−34 kg ⋅ m/s
- 6.626 × 10-23 kg ⋅ m/s
The momentum of light is exactly reversed when reflected straight back from a mirror, assuming negligible recoil of the mirror. Thus the change in momentum is twice the initial photon momentum. Suppose light of intensity 1.00 kW/m2 reflectsfrom a mirror of area 2.00 m2 each second. Using the most general form of Newton’s second law, what is the force on the mirror?
- 1.33 × 10-5 N
- 1.33 × 10−6 N
- 1.33 × 10−7 N
- 1.33 × 10−8 N
Performance Task
21.3 The Dual Nature of Light
Our scientific understanding of light has changed over time. There is evidence to support the wave model of light, just as there is evidence to support the particle model of light.
- Construct a demonstration that supports the wave model of light. Note—One possible method is to use a piece of aluminum foil, razor blade, and laser to demonstrate wave interference. Can you arrange these materials to create an effective demonstration? In writing, explain how evidence from your demonstration supports the wave model of light.
- Construct a demonstration that supports the particle model of light. Note—One possible method is to use a negatively charged electroscope, zinc plate, and three light sources of different frequencies. A red laser, a desk lamp, and ultraviolet lamp are typically used. Can you arrange these materials to demonstrate the photoelectric effect? In writing, explain how evidence from your demonstration supports the particle model of light.