12.Basic paper consists of many transparent fibers of cellulose, the main chemical in wood and cotton. Why does paper appear white, and why does it become relatively clear when you get it wet?
E.12Some light reflects from each randomly oriented interface between air and cellulose. This light ends up traveling in all directions because of the random orientations of those reflecting surfaces. When water fills the gaps between fibers, the changes in light speed are smaller and so are the reflections.
13.On a rainy day you can often see oil films on the surfaces of puddles. Why do these films appear brightly colored?
E.13Light reflects from the top and bottom surfaces of an oil film, and the two reflections interfere with one another. The type of interference depends on the film’s thickness and the light’s wavelength.
14.When two sheets of glass lie on top of one another, you can often see colored rings of reflected light. How do the nearby glass surfaces cause these colored rings?
E.14Partial reflections of light from the back of one sheet of glass and from the front of the next sheet of glass interfere with one another. Because the type of interference, constructive or destructive, depends on the spacing between the glass and the wavelength of the light, the interfering light tends to have a colored appearance.
15.If you’re wearing polarizing sunglasses and want to see who else is wearing polarizing sunglasses, you only have to turn your head sideways and look to see which people now have sunglasses that appear completely opaque. Why does this test work?
E.15Polarizing sunglasses normally block horizontally polarized light, so when you look at someone's eyes when they are wearing polarizing sunglasses, you see only vertically polarized light. If you wear polarizing sunglasses and tip your head sideways, then your sunglasses will block vertically polarized light. You will see no light coming from the eyes of other people wearing polarizing sunglasses.
16.Why is it easier to see into water when you look directly down into it than when you look into it at a shallow angle?
E.16Horizontally polarized light reflects more strongly at shallow angles than at right angles.
17.Light near 480 nm has a color called cyan. What mixture of the primary colors of light makes you perceive cyan?
E.17Green and blue.
18.What is different about the two mixtures of red and green lights that make you see yellow and orange, respectively?
E.18If the green light is relatively strong compared to the red light, you’ll see yellow. However, if the red light is relatively strong compared to the green light, you’ll see orange.
19.What colors of light does red paint absorb?
E.19The green, blue, and violet end of the spectrum.
20.What colors of light does yellow paint absorb?
E.20Blue light (and other light near the blue end of the optical spectrum).
21.If you illuminate red paint with pure blue light, what color will that paint appear?
22.Fancy makeup mirrors allow users to choose either fluorescent or incandescent illumination to match the lighting in which they’ll be seen. Why does the type of illumination affect their appearances?
E.22Since incandescent light has less blue than fluorescent light, incandescent light reflected from a person's skin would also have less blue in it. Skin can't reflect light that isn't there.
23.While a sodium atom is in its ground state, it cannot emit light. Why not?
E.23There is no lower energy state to which it can make a transition and, while it remains in the ground state, its electrons are in standing waves and cannot emit electromagnetic waves.
24.When a sodium atom is in its lowest energy excited state, it can emit light. Why?
E.24The sodium atom can undergo a radiative transition to its ground state, thereby emitting a photon of (yellow) light.
25.You expose a gas of argon atoms to light with photon energies that don’t correspond to the energy difference between any pair of states in the argon atom. Explain what happens to the light.
E.25Nothing happens because the atoms have no radiative transitions that can absorb photons of that light.
26.A discharge in a mixture of gases is more likely to emit a full white spectrum of light than a discharge in a single gas. Why?
E.26The more different atoms and molecules present in a gas discharge, the more variety there is in the possible radiative transitions and the more likely that a rich, full spectrum of white light will be emitted by the discharge.
27.If the low-pressure neon vapor in a neon sign were replaced by low-pressure mercury vapor, the sign would emit almost no visible light. Why not?
E.27Excited mercury atoms emit primarily invisible ultraviolet light.
28.Increasing the power to an incandescent bulb makes its filament hotter and its light whiter. Why doesn’t increasing the power to a neon sign change its color?