REFLECTION AND REFRACTION

29.1 Reflection
When a wave reaches a boundary between two media, usually
some or all of the wave bounces back into the first medium. The
return of a wave back into its original medium is called reflection.
Suppose you fasten a spring to a wall and send a pulse along the
spring’s length, as illustrated in Figure 29.1. The wall is a very rigid
medium compared with the spring. As a result, all the wave energy is
reflected back along the spring rather than transmitted into the wall.
Waves that travel along the spring are almost totally reflected at the wall.
If the wall is replaced with a less rigid medium, such as the heavy
spring shown in Figure 29.2, some energy is transmitted into the new
medium. Some of the wave energy is still reflected. The incoming
wave is partially reflected.
A metal surface is rigid to light waves that shine upon it. Light
energy does not propagate into the metal, but instead is returned in
a reflected wave. The wave reflected from a metal surface has almost
the full intensity of the incoming wave, apart from small energy
losses due to the friction of the vibrating electrons in the surface.
This is why metals such as silver and aluminum are so shiny. They
reflect almost all the frequencies of visible light.
Other materials such as glass and water are not as rigid to light
waves. When light shines perpendicularly on the surface of still water,
about 2% of its energy is reflected and the rest is transmitted. When
light strikes glass perpendicularly, about 4% of its energy is reflected.
Except for slight losses, the rest is transmitted.
CONCEPT
CHECK
......
What happens when a wave reaches a boundary
between two media?
CHA
29.2 The Law of Reflection
In one dimension, reflected waves travel back in the direction from
which they came. Let a ball drop to the floor, and it bounces straight
up along its initial path. In two dimensions, the situation is a little
different. A pool ball hitting the side of a pool table at an angle bounces
back at the same angle in a new direction. Likewise with light.
The direction of incident and reflected waves is best described
by straight-line rays. Incident rays and reflected rays make equal
angles with a line perpendicular to the surface, called the normal,
as shown in Figure 29.3. The angle between the incident ray and the
normal, called the angle of incidence, is equal to the angle between
the reflected ray and the normal, called the angle of reflection.
angle of incidence  angle of reflection
The law of reflection describes the relationship between the angle
of incidence and angle of reflection. The law of reflection states
that the angle of incidence and the angle of reflection are equal to
each other. The incident ray, the normal, and the reflected ray all
lie in the same plane. The law of reflection applies to both partially
reflected and totally reflected waves.
CONCEPT
CHECK
......
What is the law of reflection?
29.3 Mirrors
Consider a candle flame placed in front of a plane (flat) mirror. Rays
of light leaving the candle are reflected from the mirror surface in all
directions. The number of rays is infinite, and every one obeys the law
of reflection. Figure 29.4 shows only two rays that originate at the tip
of the candle flame and reflect from the mirror to your eye. Note that
the rays diverge (spread apart) from the tip of the flame, and continue
diverging from the mirror upon reflection. These divergent rays appear
to originate from a point located behind