Conceptual Physics Chapter 29: Reflection and Refraction

Light Striking a Metal Surface
Almost all the of the energy is reflected back.

Light Striking Glass or Water
Some of the energy is reflected and some is transmitted through glass or water.
Glass: 4% is reflected
Water: 2% is reflected

When light strikes a flat mirror at a certain angle, predict the path of the reflected light.
Reflects at the same angle but in the opposite direction.

When you view your image in a plane mirror, how far behind the mirror is your image compared with your distance in front of the mirror?
Same distance behind the mirror as the object is in front.
Object distance = image distance.

Does the law of reflection hold for diffuse reflection?
Yes. Every individual ray follows the law of reflection. But incident rays hit many different-angled surfaces which causes reflection in many different directions.

Contrast the reflection of light from a rough and a smooth surface.
Rough: Rays of light striking this surface encounter many different flat surfaces facing in all directions so light rays are reflected in many different directions.
Smooth: Rays of light strike at all the same angle and therefore reflect at all the same directions.

What is meant by the idea that a surface may be polished for some waves and rough for others?
If the surface is less than 1/8 the length of the light wave, the surface is considered smooth.
A piece of paper is considered smooth for a radio wave, but rough for a short light wave.

What is the effect on sound when room surfaces are too reflective?
Garbled sound

What is the effect on sound when room surfaces are too absorbent?
Sound is dull and lifeless.

What could an acoustical engineer do in designing a room to reduce the sound level?
Could use soft, absorbent materials so that some of the sound waves are absorbed.

How are reflection and refraction alike? How are they different?
Alike:
-occur when waves reach a boundary of a new medium.
-involve change in direction of a wave.
Different:
-reflection occurs when the waves do not go through the new medium but bounce back, refraction occurs when the wave goes into the new medium.
-refraction: waves change in speed
-reflection: waves travel at the same speed.

“Pivot”?
Wave pivots occur because of the part of the wave that reaches the new medium first changes peed while the rest of the wave does not.

What is the orientation of a ray in relation to the wave front of a wave?
A ray is perpendicular to the wave front.

Why do sounds carry across a lake better at night than during the day?
Air near the ground is colder than the air above, so speed of sound is slower near the ground and faster in the warm air above causing the sound waves to bend toward the ground.

What are 3 conditions that could cause refraction?
Light waves change speed (refract) when light passes:
-from one medium to another
-through different temperatures of the same medium
-through different densities of the same medium

Does refraction occur for both sound waves and light waves?
YES!

If light had the same speed in air and in water, would light be refracted in passing from air into water?
No because refraction is caused by change in speed so…
-No change in speed = no refraction

If you can see the face of a friend who is underwater, can she also see you?
Yes because the light paths are reversible for reflection and refraction.

Does refraction tend to make objects submerged in what seem shallower or deeper than they really are?
SHALLOWER!!

What happens to the speed of light as it goes from air to water?
Air –> water: light slows down

Which way does light bend as it passes form air to water?
Air –> water: light bends toward the normal

As light passes through glass or water, do the high or low frequencies of light interact more in the process of absorption and re-emission and therefore, slow down more?
High frequencies- b/c they are closer to the natural frequency of glass and water, and therefore resonate more.
-More interaction = Slower speed

Why does blue light refract at greater angles than red light in transparent materials?
Blue light (higher frequency) refracts (bends) more because it slows down more than red light.

Describe the condition necessary for a motorist to see a mirage.
The air near the surface must be very hot – light speeds up in the hot air and refracts upward.

Explain how a prism separates white light into colors.
Blue light slows down more than red so blue light is bent more than red. When this happens twice (at the two non-parallel glass surfaces of a prism) the effect is enough to see the rays dispersed.

Describe the sky conditions necessary for a rainbow. Where must you be to see the rainbow?
The sun must be shining in one part of the sky and water droplets from a cloud or falling rain in the opposite part of the sky. You must be standing with your back to the sun.

What are two factors that cause a diamond to “sparkles” and produce brilliant colors?
1. Small critical angles of the diamond (24.6 degrees)
2. Unusually low speed of light in diamonds (0.4 c)
-These two factors cause light to be dispersed in a broad, intense sparkle.

Compare the critical angles of water, glass and diamonds.
Water – 48 degrees
Glass – 43 degrees
Diamonds – 24. 6 degrees

How much light leaves a medium if it strikes the boundary at an angle greater than the critical angle?
NONE!!! When the angle of incidence is greater than the critical angle, all the light undergoes reflection. TOTAL INTERNAL REFLECTION.

Give 3 specific examples of uses for optical fibers.
Mechanics use them to look inside engines. Physicians used them to look inside a patient’s body.
Communications: replace copper telephone wires and underseas cables (more information can be carried on by high frequency light than by low frequency electric current.)

Image
Likeness or reproduction of an object.

Orderly Pattern
Is needed to form an image, so the light must bounce of in an orderly pattern.

Converge
A term that means “bring light rays together”.

Diverge
A term that means “spread light rays”.

Plane Mirror
A flat mirror. Produces a virtual image, of equal size and distance.

Convex Mirror
Mirror that spreads light rays. Produces a virtual images, of smaller size and farther away distance.

Concave Mirror
Mirror that focuses light rays to a point. Produces a virtual image or larger size and closer distance.

Reflection
The bouncing back of a particle or wave that strikes the boundary between two media.
-Close-up: Produces a virtual image of larger size and closer distance.
-Far-away: Produces a real image of a smaller size (depending), of farther distance.

Normal
A line perpendicular to the surface.

Angle of Incidence
Angle between an incident ray and the normal to a surface.

Angle or Reflection
Angle between a reflected ray and the normal to a surface.

Law of Reflection
The angle of incidence for a wave that strikes a surface is equal to the angle of reflection.This is true for both partially and totally reflected waves.

Virtual Image
An image formed through reflection or refraction that can be seen by an observer but cannot be projected on a screen because light from the object does not actual come to a focus.

Diffuse Reflection
The reflection of waves in many directions from a rough surface.

Reverberation
Resistance of a sound, as in an echo, due to multiple reflections.

Refraction
The change in direction of a wave as it crosses the boundary between two media in which the wave travels at different speeds.

Wave Front
The crest, trough, or any continuous portion of a two-dimensional or three-dimensional wave in which the vibrations are all the same way at the same time.

Mirage
A floating image that appears in the distance and is due to the refraction of light in Earth’s atmosphere.

Dispersion
The separation of light into colors arrange according to their frequency, by interaction with a prism or diffraction grating, for example.

Critical Angle
The minimum mass of fissionable material in a nuclear reactor or nuclear bomb that will sustain a chain reaction.

Total Internal Reflection
The 100%% reflection (with no transmission) of light that strikes the boundary between two media at an angle greater than the critical angle.

Optical Fiber
A transparent fiber, usually of glass or plastic, that can transmit light down its length by means of total internal reflection.

Real Image is…
formed by intersecting light rays and is always upside down.

Virtual Image is…
an image that appears to be in a location where light does not rally reach and is always right-side up.

Convex Mirror
Produces a smaller, upright image.

Concave Mirror
Produces a larger, upright image/ produces an inverted image (which may be smaller or larger) depending on distance from the mirror.

Security Mirror
Convex

Flashlight
Concave

Cosmetic Mirror
Concave

Rear-View Mirror
Convex

Multiple Reflections of Sound Waves
Reverberations

Two Conditions that cause Sound Waves to become Refracted
a. Uneven Winds
b. Uneven Temperatures