Conceptual Physics Fundamentals

Paul G. Hewitt

Chapter 15

Quantum Theory - all with Video Answers

Educators

Chapter Questions

Problem 1

Which has more energy- a photon of visible light or a photon of ultraviolet light?

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Problem 2

We speak of photons of red light and photons of green light. Can we speak of photons of white light? Why or why not?

Zulfiqar Ali

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Problem 3

Which laser beam carries more energy per photon-a red beam or a green beam?

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Problem 4

If a beam of red light and a beam of blue light have exactly the same energy, which beam contains the greater number of photons?

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Problem 5

If we double the frequency of light, we double the energy of each of its photons. If we instead double the wavelength of light, what happens to the photon energy?

Eduard Sanchez

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Problem 6

Do phosphors on the inside of fluorescent lamps convert ultraviolet light to visible light, or visible light to ultraviolet light? Defend your answer.

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Problem 7

Silver bromide (AgBr) is a light-sensitive substance used in some types of photographic film. To cause exposure of the film, it must be illuminated with light having sufficient energy to break apart the molecules. Why do you suppose this film may be handled without exposure in a darkroom illuminated with only red light? How about blue light? How about very bright red light relative to very dim blue light?

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Problem 8

Sunburn produces cell damage in the skin. Why is ultraviolet radiation capable of producing this damage, while visible radiation, even if more intense, is not?

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Problem 9

In the photoelectric effect, does brightness or frequency determine the kinetic energy of the ejected electrons? Which determines the number of the ejected electrons?

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Problem 10

A very bright source of red light has much more energy than a dim source of blue light, but the red light has no effect in ejecting electrons from a certain photosensitive surface. Why is this so?

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Problem 11

Why are ultraviolet photons more effective at inducing the photoelectric effect than photons of visible light?

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Problem 12

Why does light striking a metal surface eject only electrons, not protons?

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Problem 13

Does the photoelectric effect depend on the wave nature or the particle nature of light?

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Problem 14

What is the evidence for the claim that iron exists in the relatively cool outer layer of the Sun?

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Problem 15

What difference does an astronomer see between the emission spectrum of an element in a receding star and a spectrum of the same element in the lab?

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Problem 16

A blue-hot star is about twice as hot as a red-hot star. But the temperatures of the gases in advertising signs are about the same, whether they emit red or blue light. What is your explanation?

Eduard Sanchez

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Problem 17

Does atomic excitation occur in solids as well as in gases? How does the radiant energy from an incandescent solid differ from the radiant energy emitted by an excited gas?

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Problem 18

If atoms of a substance absorb ultraviolet light and emit blue light, what becomes of the "missing" energy?

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Problem 19

When an electron makes a transition from its first quantum level to ground level, the energy difference is carried by the emitted photon. In comparison, how much energy is involved to return an electron at ground level to the first quantum level?

Guilherme Barros

Guilherme Barros

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Problem 20

Your friend reasons that, if ultraviolet light can activate the process of fluorescence, infrared light ought to also. Your friend looks to you for approval or disapproval of this idea. What is your position?

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Problem 21

The forerunner to the laser involved microwaves rather than visible light. What does maser mean?

Eduard Sanchez

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Problem 22

The first laser consisted of a red ruby rod activated by a photoflash tube that emitted green light. Why would a laser composed of a green crystal rod and a photoflash tube that emits red light not work?

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Problem 23

A laboratory laser has a power of only $0.8 \mathrm{~mW}$ $\left(8 \times 10^{-4} \mathrm{~W}\right)$. Why does it seem more powerful than light from a 100 -W lamp?

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Problem 24

How do the avalanches of photons in a laser beam differ from the hordes of photons emitted by an incandescent lamp?

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Problem 25

A friend speculates that scientists in a certain country have developed a laser that produces far more energy than is put into it. Your friend asks for your response to this speculation. What is your response?

Eduard Sanchez

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Problem 26

Since every object has some temperature, every object radiates energy. Why, then, can't we see objects in the dark?

Ben Nicholson

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Problem 27

If we continue heating a piece of initially room-temperature metal in a dark room, it will begin to glow visibly. What will be its first visible color, and why?

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Problem 28

We can heat a piece of metal to red hot and then to white hot. Can we heat it until the metal glows blue hot? And would it be solid at this temperature?

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Problem 29

How do the surface temperatures of reddish, bluish, and whitish stars compare?

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Problem 30

Part a in the sketch on the next page shows a radiation curve of an incandescent solid and its spectral pattern as produced with a spectroscope. Part b shows the "radiation curve" of an excited gas and its emission spectral pattern. Part c shows the curve produced when a cool gas is between an incandescent source and the viewer; the corresponding spectral pattern is left as an exercise for you to construct. Part d shows the spectral pattern of an incandescent source as seen through a piece of green glass; you are to sketch in the corresponding radiation curve.

Morgan Cheatham

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Problem 31

Consider just four of the energy levels in a certain atom, as shown in the diagram. How many spectral lines will result from all possible transitions among these levels? Which transition corresponds to the highest-frequency light emitted? To the lowest-frequency light emitted?
$n=4$_____
$n=3$_____
$n=2$_____
$n=1$_____

Ben Nicholson

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Problem 32

An electron de-excites from the fourth quantum level in the diagram above to the third and then directly to the ground state. Two photons are emitted. How does the sum of their frequencies compare with the frequency of the single photon that would be emitted by de-excitation from the fourth level directly to the ground state?

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Problem 33

When do photons behave like a wave? When do they behave like particles?

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Problem 34

Light has been argued to be a wave and then a particle, and then back again. Does this indicate that light's true nature probably lies somewhere between these two models?

Eduard Sanchez

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Problem 35

What laboratory device utilizes the wave nature of electrons?

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Problem 36

When a photon hits an electron and gives it energy, the photon has less energy after it bounces from the electron. What happens to the frequency of the photon after it bounces from the electron? (This phenomenon is called the Compton effect.)

Mayank Tripathi

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Problem 37

An electron and a proton travel at the same speed. Which has more momentum? Which has the longer wavelength?

Eduard Sanchez

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Problem 38

One electron travels twice as fast as another. Which has the longer wavelength?

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Problem 39

Does the de Broglie wavelength of a proton become longer or shorter as its velocity increases?

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Problem 40

We don't notice the wavelength of moving matter in our common experience. Is this because the wavelength is extraordinarily large or extraordinarily small?

Mayank Tripathi

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Problem 41

What principal advantage does an electron microscope have over an optical microscope?

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Problem 42

A friend says, "If an electron is not a particle, then it must be a wave." What is your response? (Do you hear "eitheror" statements like this often?)

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Problem 43

Consider one of the many electrons on the tip of your nose. If somebody looks at it, will its motion be altered?
How about if it is looked at with one eye closed? With two eyes open, but crossed? Does Heisenberg's uncertainty principle apply here?

Keshav Singh

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Problem 44

Does the uncertainty principle tell us that we can never know anything for certain?

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Problem 45

Do we inadvertently alter the realities that we attempt to measure in a public opinion survey? Does Heisenberg's uncertainty principle apply here?

Mayank Tripathi

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Problem 46

If the behavior of a system is measured exactly for some period of time and is understood, does it follow that the future behavior of that system can be exactly predicted? (Is there a distinction between properties that are measurable and properties that are predictable?)

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Problem 47

If a butterfly causes a tornado, does it make sense to eradicate butterflies? Defend your answer.

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Problem 48

We hear the expression "taking a quantum leap" to describe large changes. Is the expression appropriate? Defend your answer.

Zulfiqar Ali

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Problem 49

What is it that waves in the Schrödinger wave equation?

Keshav Singh

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Problem 50

If the world of the atom is so uncertain and subject to the laws of probabilities, how can we accurately measure such things as light intensity, electric current, and temperature?

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Problem 51

What evidence supports the notion that light has wave properties? What evidence supports the view that light has particle properties?

Eduard Sanchez

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Problem 52

When and where do Newton's laws of motion and quantum mechanics overlap?

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Problem 53

What does Bohr's correspondence principle say about quantum mechanics versus classical mechanics?

Eduard Sanchez

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Problem 54

Richard Feynman in his book The Character of Physical Law states: "A philosopher once said, 'It is necessary for the very existence of science that the same conditions always produce the same results.' Well, they don't!" Who was speaking of classical physics, and who was speaking of quantum physics?

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Problem 55

To measure the exact age of Old Methuselah, the oldest living tree in the world, a Nevada professor of dendrology, aided by an employee of the U.S. Bureau of Land Management, cut the tree down in 1965 and counted its rings. Is this an extreme example of changing that which you measure or an example of arrogant and criminal stupidity?

Keshav Singh

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