Chapter 29 Light Waves

When the reflected path from one surface of a thin film is one-half wavelength different in length from the reflected path from the other surface and no phase change occurs, will the result be destructive interference or constructive interference?

Destructive interference will result.

Is diffraction more pronounced through a small opening or through a large opening?

Diffraction is more pronounced through a small opening.

Why can sunlight that illuminates Earth be approximated by plane waves, whereas the light from a nearby lamp cannot?

Earth intercepts such a tiny fraction of the expanding spherical wave from the Sun that it can be approximated as a plane wave (just as a small portion of the spherical surface of Earth can be approximated as flat). The spherical waves from a nearby lamp have noticeable curvature. (See Figures 29.3 and 29.4)

According to Huygens, how does every point on a wavefront behave?

Every point on a wavefront behaves as a source of new wavelets, which combine to form new wavefronts.

Huygens' principle

Every point on a wavefront may be considered the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the waves.

Why is it important that monochromatic light be used?

If light of various wavelengths were diffracted by the slits, dark fringes for one wavelength would be filled in with bright fringes for another, resulting in no distinct fringe pattern. If you haven't seen this, be sure to ask your instructor to demonstrate it.

Why may an ideal Polaroid filter transmit anything from zero to 100% of incident polarized light?

If the filter is aligned with the polarization of the light, all the light gets through. If it is aligned perpendicular to the polarization of the light, none gets through. At any other angle, some of the light gets through because the polarized light can be "resolved" (like a vector) into components parallel and perpendicular to the alignment of the filter.

The colored wings of many butterflies are due to pigmentation, but in others, such as the Morpho butterfly, the colors do not result from any pigmentation. When the wing is viewed from different angles, the colors change. How are these colors produced?

Interference colors result from double reflections from the upper and lower surfaces of the thin transparent coating on the butterfly wings. Some other butterfly wings produce colors by diffraction, where ridges in the surface act as diffraction gratings.

Which of these is most central to holography: interference, selective reflection, refraction, or all of these?

Interference is central to holography.

When dishes are not properly rinsed after washing, different colors are reflected from their surfaces. Explain.

Interference of light from the upper and lower surfaces of the soap or detergent film is occurring.

What produces iridescence?

Iridescence is produced by light interference.

What is the polarization of light?

It is a property of waves by which oscillation occurs in a particular orientation. In the case of light which is composed of an electric field and a magnetic field, the polarization of the electric field refers to the oscillation of the electric field either in a plane or in a circle.

Which will give wider-spaced fringes in a double-slit experiment: red light or violet light? (Let Figure 29.18 guide your thinking.)

Longer-wavelength red light will give wider-spaced fringes.

Which more easily diffracts around buildings: AM or FM radio waves? Why?

Longer-wavelength waves diffract more than shorter waves, so the longer waves of AM radio diffract more.

If the double slits were illuminated with monochromatic (single-frequency) red light, would the fringes be more widely or more closely spaced than if they were illuminated with monochromatic blue light?

More widely spaced. Can you see in Figure 29.18 that a slightly longer---and therefore a slightly more displaced---path from entrance slit to screen would result for the longer waves of red light?

What role do polarization filters play in 3-D projection?

Polarization filters at right angles to each other project a pair of images that merge on a screen. These images can reach separate eyes when the screen is viewed through polarization filters at the same right angles to each other.

Polarized light is a part of nature, but polarized sound is not. Why?

Polarization is a property of transverse waves. Unlike light, sound is a longitudinal wave and can't be polarized. Whether a wave can be polarized or not, in fact, is one of the tests to distinguish transverse waves from longitudinal waves.

To remove the glare of light from a polished floor, should the axis of a Polaroid filter be horizontal or vertical?

The axis of the filter should be vertical, not allowing the passage of the glare, which is parallel to the plane of the floor---horizontal.

Diffraction

The bending of light that passes near the edge of an object or through a narrow slit, causing the light to spread.

What happens to the distance between interference fringes if the separation between two slits is increased?

The fringes become closer together as the slits are moved farther apart. (Note this in the photos of Figure 29.14.)

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

The 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 the light from a single source divides and recombines.

Superposition

The overlapping and combining of waves.

The digital displays of watches and other devices are normally polarized. What related problem can occur when you are wearing Polaroid sunglasses?

The problem is serious: Depending on the orientation of the polarization axes of the display and the glasses, no display may be seen.

When ordinary light is incident at an oblique angle upon water, what can you say about the reflected light?

The reflected light is polarized in the direction of the plane surface of reflection.

Interference

The result of superposing different waves, usually of the same wavelength. Constructive interference results from crest-to-crest reinforcement; destructive interference results from crest-to-crest cancellation. The interference of selected wavelengths of light produces colors know as interference colors.

How would the spacings between Newton's rings differ when illuminated by red light and by blue light?

The rings would be more widely spaced for the longer-wavelength red light than for the shorter waves of blue light. Do you see the geometric reason for this?

If the path-length difference between two identical and coherent beams is two wavelengths when they arrive on a screen, will they produce a dark or a bright spot?

The spot will be bright due to constructive interference.

Why do radio waves diffract around buildings but light waves do not?

The wavelengths of AM radio waves are hundreds of meters, much greater than the size of buildings, so they are easily diffracted around buildings. FM wavelengths are a few meters, borderline for diffraction around buildings. Light, with wavelengths a tiny fraction of a centimeter, show no appreciable diffraction around buildings.

Why does a microscopist use blue light rather than white light to illuminate objects being viewed?

There is less diffraction with shorter-wavelength blue light, so the microscopist sees more detail (just as a dolphin beautifully investigates fine detail in its environment by the echoes of ultra-short wavelengths of sound).

What accounts for the different colors in either a soap bubble or a layer of gasoline on water?

These colors are interference colors, resulting from double reflection from two surfaces.

By how much should a pair of light rays from a common source differ in distance traveled to produce destructive interference?

They should differ by a half wavelength, or an odd number of half wavelengths.

What aspect of light did Thomas Young demonstrate in his famous light experiment?

Thomas Young demonstrated the wave nature of light.

Why will light pass through a pair of Polaroids when the axes are aligned but not when the axes are at right angles to each other?

When the axes are aligned, what gets through one gets through the other. When they are at right angles, what gets through the first is absorbed by the second.

Does parallax underlie the depth perceived in stereo views?

Yes. The phenomenon of parallax does underlie depth perception.

How can a single sheet of Polaroid film be used to show that the sky is partially polarized? (Interestingly enough, unlike humans, bees and many insects can discern polarized light and use this ability for navigation.)

You can determine that the sky is partially polarized by rotating a single sheet of Polaroid film in front of your eye while viewing the sky. You'll notice the sky darken when the axis of the Polaroid is perpendicular to the polarization axis of the skylight.

How can you determine the polarization axis for a single sheet of Polaroid (especially if you're at the edge of a lake)?

You can determine the polarization axis by viewing the glare from a flat surface, as in Figure 29.34. The glare is most intense when the polarization axis is parallel to the flat surface.

Hologram

A two-dimensional microscopic interference pattern that shows three-dimensional optical images.

A double-slit arrangement produces interference fringes for yellow sodium light. Should red light or blue light be used to produce narrower-spaced fringes?

Blue light will produce narrower-spaced fringes.

Because of wave interference, a film of oil on water in sunlight is seen to be yellow to observers directly above in an airplane. What color of light transmits through the oil (that would be seen by a scuba diver directly below)?

Blue, the complementary color, transmits. The blue is white minus the yellow light that is seen above. (Note that this exercise goes back to information in Chapter 27.)

What is meant by saying that a surface is optically flat?

An optically flat surface is one where interference fringes are uniform in shape.

In which of these is color formed by refraction: flower petals, rainbows, soap bubbles? By selective reflection? By thin-film interference?

Refraction: rainbow. Selective reflection: flower petals. Thin-film interference: soap bubbles.

Polarization

The alignment of the transverse electric vibrations of electromagnetic radiation. Such waves of aligned vibrations are said to be polarized.