Physics chp 26-32 Game

If, while standing on a bank, you wish to spear a small blue fish beneath the water surface in front of you, should you aim above, below, or directly at the observed fish to make a direct hit? If, instead, you zap the fish with a red laser, should you aim above, below, or directly at the observed fish? Defend your answers.

The angle of refraction for blue light is greater than for red, so if you fired your red beam along the line of sight for blue, the beam would pass above the fish. So you should aim slightly below the sighted fish.

What colors of ink do color ink-jet printers use to produce a full range of colors? Do the colors form by color addition or by color subtraction?

The colors used are magenta, cyan, yellow, and magenta. Black is also used. Colors are formed by color subtraction.

What is the fundamental source of electromagnetic radiation?

The fundamental source of EM radiation is oscillating (accelerating) electric charges.

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

The kinetic energy of ejected electrons depends on the frequency of the illuminating light. With sufficiently high frequency, the number of electrons ejected in the photoelectric effect is determined by the number of photons incident upon the metal. So whether or not ejection occurs depends on frequency, and how many electrons are ejected depends on the brightness of the sufficiently high-frequency light.

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?

The laws of probability applied to one or a few atoms give poor predictability, but for hordes of atoms, the situation is entirely different. Although it is impossible to predict which electron will absorb a photon in the photoelectric effect, it is possible to predict accurately the current produced by a beam of light on photosensitive material. W

How can a hydrogen atom, which has only one electron, have so many spectral lines?

The many spectral lines from the element hydrogen are the result of the many energy states the single electron can occupy when excited. Also remember that the emission tube has more than one hydrogen atom, and the different electrons in the different atoms are not all moving to the same excited states at the same time.

How does the amplitude of a matter wave relate to probability?

The matter wave is represented by the symbol Ψ (psi), known as the wave function. Where ever Ψ is large (i.e. the amplitude is large), the particle is more likely to be found. Where Ψ is small, the particle is less likely to be found. (The actual probability is proportional to psi-squared, |Ψ|2.)

Why do the iridescent colors seen in some seashells (such as abalone shells) change as the shells are viewed from various positions?

The optical paths of light from upper and lower reflecting surfaces change with different viewing positions. Thus, a change in color can be seen by tilting the shell at different angles.

In terms of wavelength, what is the smallest orbit that an electron can have about the atomic nucleus?

The smallest orbit would be one with a circumference equal to one wavelength, according to the de Broglie model.

Car mirrors are uncoated on the front surface and silvered on the back surface. When the mirror is properly adjusted, light from behind reflects from the silvered surface into the driver's eyes. Good. But this is not so good at nighttime with the glare of headlights behind. This problem is solved by the wedge shape of the mirror (see sketch). When the mirror is tilted slightly upward to the "nighttime" position, glare is directed upward toward the ceiling, away from the driver's eyes. Yet the driver can still see cars behind in the mirror. Explain.

Two surfaces of the mirror reflect light. The front surface reflects about 4% of incident light, and the silvered surface reflects most of the rest. When the mirror is tilted in the "daytime" position, the driver sees light reflecting from the silvered surface. In the "nighttime" position, with the mirror tilted upward, light reflecting from the silvered surface is directed above the driver's view and the driver sees light reflected from the front surface of the mirror. That 4% of rearview light is adequate for night driving.

Does the photoelectric effect prove that light is made of particles? Do interference experiments prove that light is composed of waves? (Is there a distinction between what something is and how it behaves?)

We can never definitely say what something is, only how it behaves. Then we construct models to account for the behavior. The photoelectric effect doesn't prove that light is corpuscular, but supports the corpuscular model of light. Particles best account for photoelectric behavior. Similarly, interference experiments support the wave model of light. Waves best account for interference behavior. We have models to help us conceptualize what something is; knowledge of the details of how something behaves helps us to refine the model. It is important that we keep in mind that our models for understanding nature are just that: models. If they work well enough, we tend to think that the model represents what is.

Why can't we see stars in the daytime?

We cannot see stars in the daytime because their dim light is overwhelmed by the brighter skylight, which is sunlight scattered by the atmosphere.

What is wrong with the cartoon of the man looking at himself in the mirror? (Have a friend face a mirror as shown, and you'll see.)

We would not see an image of the man in the mirror as shown. If he is viewing himself, then we wouldn't also be able to see his image unless we were in back (or in front) of him. If we are to stand to the side of the man and see him and an image of him in the mirror, then the mirror cannot be exactly in front of him. The mirror would have to be located to the man's right, as shown in the sketch. The man's view would miss the mirror completely.

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

When a spectrum of the Sun is compared with the spectrum of the element iron, the iron lines overlap and perfectly match certain Fraunhofer lines. This is evidence for the presence of iron in the Sun.

A lamp filament is made of tungsten. Why do we get a continuous spectrum rather than a tungsten line spectrum when light from an incandescent lamp is viewed with a spectroscope?

When tungsten atoms are close-packed in a solid, the otherwise well-defined energy levels of outer electron shells are smeared by mutual interactions among neighboring atoms. The result is an energy band composed of myriad separate levels very close together. Because there are about as many of these separate levels as there are atoms in the crystalline structure, the band cannot be distinguished from a continuous spread of energies.

Yellow light + blue light = _______ light Green light + _______ light = white light Magenta + yellow + cyan = _______ light

White. Magenta. White.

What percentage of light is transmitted by two ideal Polaroids, one on top of the other with their polarization axes aligned? With their axes at right angles to each other?

With polarization axes aligned, a pair of Polaroids will transmit all components of light along the axes. That's 50%, as explained in the preceding answer. Half of the light gets through the first Polaroid, and all of that gets through the second. With axes at right angles, no light will be transmitted.

When you look at a distant galaxy through a telescope, how is it that you're looking backward in time?

You're seeing the galaxies as they "were" when light left them, long, long ago.

When does a photon behave like a wave? When does it behave like a particle?

A photon behaves like a wave when traveling from one place to another (i.e. photons interacting with each other), and like a particle when it is emitted or absorbed my matter (i.e. photons interacting with charged particles).

Only some of the people on the daytime side of Earth can witness a solar eclipse when it occurs, whereas all the people on the nighttime side of Earth can witness a lunar eclipse when it occurs. Why is this so?

A solar eclipse is a shadow of the Moon that reaches a relatively small part of the Earth, and only those people in the shadow or partial shadow experience it. But a lunar eclipse is the Earth's shadow upon the Moon, which is visible to all who can see the Moon.

Your friend reasons that magenta and yellow paint mixed together will produce red because magenta is a combination of red and blue and yellow is a combination of red and green - and that the color in common is red. Do you agree or disagree, and why?

Agree, for the "light mathematics" is correct.

Why does a high-flying airplane cast little or no shadow on the ground below while a low-flying airplane casts a sharp shadow?

Any shadow cast by a faraway object such as a high-flying plane is filled in mainly by light tapering in from the Sun, which is not a point source. This tapering is responsible for the umbra and penumbra of solar eclipses. If the plane is low to the ground, however, the tapering of light around the airplane may be insufficient to fill in the shadow, part of which can be seen.

Light will not pass through a pair of Polaroid sheets when they are aligned perpendicularly. However, if a third Polaroid is sandwiched between the two with its alignment halfway between the alignments of the other two (that is, with its axis making a 45° angle with each of the other two alignment axes), some light does get through. Why?

Call the three Polaroids 1, 2, and 3. The first one acts as a polarizer of the unpolarized light, ideally letting half of it through with a specific polarization direction that is perpendicular to the axis of Number 3. So when only 1 and 3 are present, no light gets through. But Number 2, when placed between 1 and 3, is illuminated by light aligned at 45 degrees to its axis, so it lets half of the light through. The light striking Number 3 is now aligned at 45° to the axis of Number 3. So Number 3 transmits half of the light that strikes it. (The amount that gets through is one-eighth of the original intensity.)

You can get a sunburn on a cloudy day, but you can't get a sunburn even on a sunny day if you are behind glass. Explain.

Clouds are transparent to ultraviolet light, which is why clouds offer no protection from sunburn. Glass, however, is opaque to ultraviolet light, and will therefore shield you from sunburn.

Why does no stable electron orbit with a circumference of 2.5 de Broglie wavelengths exist in any atom?

Constructive interference to form a standing wave requires an integral number of wavelengths around one circumference. Any number of de Broglie wavelengths not equal to a whole number would lead to destructive interference, preventing the formation of a standing wave.

What evidence can you cite for the wave nature of light? For the particle nature of light?

Diffraction, polarization, and interference are evidence of the wave nature of light; the photoelectric effect is evidence of the particle nature of light.

The intensity of light decreases as the inverse square of the distance from the source. Does this mean that light energy is lost? Explain.

Energy is spread out and diluted, but not "lost." We distinguish between something being diluted and something being annihilated. In accord with the inverse-square law, light intensity gets weaker with distance, but the total amount of light over a spherical surface is the same at all distances from the source.

Why are fabrics that fluoresce when exposed to ultraviolet light so bright in sunlight?

Fabrics and other fluorescent materials produce bright colors in sunlight because they both reflect visible light and transform some of the Sun's ultraviolet light into visible light. They literally glow when exposed to the combined visible and ultraviolet light of the Sun.

Fermat's principle is of least time rather than of least distance. Would least distance apply as well for reflection? For refraction? Why are your answers different?

Fermat's principle for refraction is of least time, but for reflection it could be of least distance as well. This is because light does not change mediums for reflection so no change in speed occurs and least-time paths and least- distance paths are equivalent.

How could you use the spotlights at a play to change the performers' clothes suddenly from yellow to black?

If the yellow clothes of stage performers are illuminated with a complementary blue light, they will appear black.

What color of light will be transmitted through overlapping cyan and magenta filters?

If they are good quality filters, blue will be produced. (Poor quality filters may produce purple.)

How does the wave model of electrons orbiting the nucleus account for discrete energy values rather than a continuous range of energy values?

If we think of electrons as orbiting the nucleus in standing waves, then the circumference of these wave patterns must be a whole number of wavelengths.

If the electron in a hydrogen atom obeyed classical mechanics instead of quantum mechanics, would it emit a continuous spectrum or a line spectrum? Explain.

It would emit a continuous spectrum. Its energy would change gradually and continuously as it spiraled inward and it would radiate at its rotational frequency, which would be continuously increasing.

Will the light from two very close stars produce an interference pattern? Explain.

Light from a pair of stars will not produce an interference pattern because the waves of light from the two separate sources are incoherent; when combined they smudge. Interference occurs when light from a single source divides and recombines.

Which will produce more widely spaced fringes of light when passed through a diffraction grating - light from a red laser or light from a green laser?

Light from a red laser.

Ultraviolet light causes sunburns, whereas visible light, even of greater intensity, does not. Why is this so?

More energy is associated with each photon of ultraviolet light than with a photon of visible light. The higher-energy ultraviolet photon can cause sunburn-producing chemical changes in the skin that a visible photon cannot.

Hold a pocket mirror almost at arm's length from your face and note how much of your face you can see. To see more of your face, should you hold the mirror closer or farther away, or would you have to have a larger mirror? (Try it and see!)

Note in your pocket mirror that the amount of your face you can see is twice the size of the mirror - whether you hold it close or at arm's length. (You can win bets on this question!)

Suppose that you lived in a hypothetical world in which you'd be knocked down by a single photon, in which matter would be so wavelike that it would be fuzzy and hard to grasp, and in which the uncertainty principle would impinge on simple measurements of position and speed in a laboratory, making results irreproducible. In such a world, how would Planck's constant compare with the accepted value?

Planck's constant would be much, much larger than its present value.

In which of these is color formed by refraction? Flower petals, rainbow, soap bubbles, the blue sky By selective reflection? By thin-film interference? By scattering?

Refraction: rainbow. Selective reflection: flower petals. Thin-film interference: soap bubbles. Scattering: the blue sky.

What does it mean to say that something is quantized?

Saying something is quantized is to say it is composed of elementary units. Electric charge, for example, is composed of multiples of the charge of the electron, so we say charge is quantized. A gram of pure gold is quantized in that it is made of a whole number of gold atoms. In this chapter we learn that light - radiant energy - is also quantized.

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?

Six transitions are possible. 2 → 1, 3 → 1, 4 → 1, 3 → 2, 4 → 2, and 4 → 3. The highest-frequency transition is from quantum level 4 to 1. The lowest-frequency transition is from quantum level 4 to 3.

Why is the lettering on the front of some vehicles "backward"?

Such lettering is seen in proper form in the rearview mirrors of cars ahead.