Physics 28 Questions

Cite the *law of reflection*.

*The angle of incidence equals the angle of reflection.*

At what angle inside glass is light totally internally reflected? At what angle inside a diamond is light totally internally reflected?

43° for glass. Light in the glass, incident at angles larger than 43° to the surface, is totally internally reflected. 24.5° for diamond. Most light incident on the sloped backsides at angles larger than 24.5° is totally internally reflected.

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!)

A larger mirror is actually needed to see more of your face! Simply bringing the pocket mirror further from your face does not "zoom out" and fit more image in the reflective surface. The image still fills out the parameters of the surface.

What kind of lens can be used to produce a real image? A virtual image?

A real image can only be made with a converging lens (light rays converge at the location of the image, can be displayed on a screen). A virtual image can be made with either a converging or diverging lens.

Does a viewer see a single color or a spectrum of colors coming from a single faraway drop?

A viewer sees a single color from a single faraway drop. "Although each drop disperses a full spectrum of colors, an observer is in a position to see the concentrated light of only a single color from any one drop."

Distinguish between a virtual image and a real image. Mention in each case whether the image made by a single lens is right-side up or inverted.

A virtual image, unlike a real image, cannot be displayed on a screen. A virtual image is upright, whereas a real image is inverted.

What is astigmatism, and how can it be corrected?

Astigmatism is caused when the cornea has a different radius of curvature in one direction compared to another. It is cured by adding a lens with a different radius of curvature in two different directions. "The cylindrical lenses have more curvature in one direction than in another."

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?

For reflection, yes, since the light stays in the same medium and has a constant speed. For refraction, no. The light enters a different medium which a different speed. the path of least time is no longer the same as the shortest path.

How does incident light that falls on an object affect the motion of electrons in the atoms of the object?

Incident light makes the electrons oscillate. The electrons then emit light or absorb the light and convert it to heat.

Does a single raindrop illuminated by sunlight deflect light of a single color or does it disperse a spectrum of colors? Does it do this by reflection, refraction, or both?

It disperses a spectrum of colors, by both reflection and refraction. "Each color is partly refracted out into the air, and partly reflected back into the water."

How does refraction change the apparent depth of a swimming pool?

It makes the pool look shallower - only ¾ as deep. "Looking straight down into water, an object submerged 4m beneath the surface appears only 3 m deep. Because of refraction, it appears to be closer, bigger."

In which medium does light travel faster - thin air or dense air? How does this affect the length of daylight hours?

It travels faster in thin air, which makes sunlight curve and extends the length of the day.

Why is vision sharpest when the pupils of the eye are very small?

Light passes only through the center of the lens, where aberrations are minimal

What is *Fermat's principle of least time*?

Light takes the quickest path in going from one place to another.

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.

Spear: Below because of refraction the speed of light is slower in water. Laser: Directly at it, the water will bend the light aiming it at the fish because of refraction.

How does the angle at which a ray of light strikes a pane of window glass compare with the angle at which it passes out the other side?

The angles are the same. The emerging light is displaced but parallel to the incident light.

What is the focal length of a lens? What is the focal point?

The distance between the center of the lens and either focal point is the focal length. The focal point is the point where a beam of parallel light, parallel to the principal axis, converges or appears to converge.

What is the relationship between index of refraction and the speed of light in a material?

The index of refraction is inversely proportional to the speed of light in a material. Index of Refraction (n) = speed of light in vacuum / speed of light in material. Snell's Law (Quantitative Law of Refraction): n1 sin θ1 = n2 sin θ2. for a vacuum, n = 1.

When the wheel of a cart rolls from a smooth sidewalk onto a plot of grass, the interaction of the wheel with the blades of grass slows the wheel. What slows light when it passes from air into glass or water?

The light oscillates electrons in the atoms of the glass or water. The electrons absorb some of the energy and reemit it after a delay at each atom.

What is meant by critical angle?

The minimum angle of incidence inside a medium where light is totally reflected. A beam of light no longer emerges into the air above the surface. Just below this angle, the intensity of the emerging beam reduces to zero and grazes the surface (fig. 28.37)

Light normally travels in straight lines, but it "bends" in an optical fiber. Explain.

Total internal reflection keeps the light trapped inside the optical fiber.

Which travels more slowly in glass, red light or violet light? Why?

Violet light travels slower because it is closer in frequency to the ultraviolet resonance of the atoms in the glass. Since the natural or resonant frequency of most transparent materials is in the ultraviolet part of the spectrum, higher-frequency light travels more slowly that lower-frequency light." Violet light travels about 1% slower in ordinary glass than red light. Light waves with colors between red and violet travel at their own intermediate speeds.

When is the angle at which a ray of light strikes glass not the same as the angle it exits?

When the light strikes the glass in one medium (such as air) and exits the glass in another medium (such as water)

Distinguish between a converging lens and a diverging lens.

When two parallel rays of light pass through a converging lens, the rays bend towards each other. When they pass through a diverging lens, the rays move apart from each other.

What fraction of the light shining straight at a piece of clear glass is reflected from the first surface?

about 4% is reflected from each surface, for normal incidence (light perpendicular to the surface). On clean polished silver surfaces, 90% incident light is reflected.

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.)

his reflection is too close to the mirror surface! The reflection would show a different portion of the face than what is depicted, more like the actual photo above it on the page.

Is a mirage the result of reflection or refraction? Why does it happen?

Refraction. Light curves through warm, low-density air near the ground, where it travels faster. "The apparent 'wetness' of the road is not a reflection of the sky by water but rather refraction of sky light through hotter, less dense air near the road surface." *Refraction is simply a consequence of light having different avg speeds in different media."

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

This is so that drivers in front of the ambulance, when viewing their rear-view mirrors, will see the correct phrase as reflected!

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

The mirror is wedge shaped; in normal position, the silvered back surface is responsible for the reflection into the driver's eyes. By tilted the wedge angle of the glass up, the silvered surface (reflective) goes up too; a dramatic reduction of brightness (glare) from the car behind. However, the driver can still see cars behind.

Relative to the distance of an object in front of a plane (flat) mirror, how far behind the mirror is the image?

The virtual image is the same distance behind the plane mirror as the object is in front of it. The virtual image and object will have the same size.

Why is a secondary rainbow dimmer than a primary bow? How are the colors arranged in the secondary bow?

There is a second reflection with a loss of light. The colors are in the opposite order of the primary bow, with red on the inside instead of the outside. "Double reflection in a drop produces a secondary bow. Because of this extra reflection (and extra refraction loss), the secondary bow is much dimmer and its colors are reversed."

What do the electrons affected by illumination do when they are made to vibrate with greater energy?

They reemit more light or collide with neighbors, thereby converting light energy to more internal energy.

Are eyeglasses made with "high index of refraction" materials thinner or thicker? Why?

Thinner. Light bends more entering high index of refraction materials. "The greater the n, the greater the bending of light in the lens, which translates into less lens thickness." The n of high-index plastic lenses reaches 1.76, so light slows more and bends more, and lenses can be made thinner- good news for nearsighted people who want thinner eyeglasses.

Does the law of reflection hold for curved mirrors?

Yes. It is as if the curved mirror is made of many small plane mirrors at slightly different orientations to each other. "At each point, the angle of incidence is equal to the angle of reflection. The normals at different points on a surface are not parallel to one another." In convex or concave mirror, the law of reflection applies.

Can a surface be considered polished for some waves and not for others?

Yes. The mesh of a radio dish is polished (good reflector) for long wavelength radio waves and yet rough for short wavelength light waves. (diffuse reflection in many directions)

What is the angle between a light ray and its wavefront?

a right angle. "The light ray remains perpendicular to the wavefront and therefore bends at the surface." Wavefront: crest, trough, any continuous portion of a wave.