1-14
If the reflecting surface is smooth and polished, the angle between the incident ray and the normal
will be the same as the angle between the reflected ray and the normal. This conforms to the law of
reflection which states: The angle of incidence is equal to the angle of reflection.
The amount of incident wave energy reflected from a given surface depends on the nature of the
surface and the angle at which the wave strikes the surface. As the angle of incidence increases, the
amount of wave energy reflected increases. The reflected energy is the greatest when the wave is nearly
parallel to the reflecting surface. When the incident wave is perpendicular to the surface, more of the
energy is transmitted into the substance and less is reflected. At any incident angle, a mirror reflects
almost all of the wave energy, while a dull, black surface reflects very little.
Q11. What is the law of reflection?
Q12. When a wave is reflected from a surface, energy is transferred. When is the transfer of energy
greatest?
Q13. When is the transfer of energy minimum?
Refraction
When a wave passes from one medium into another medium that has a different velocity of
propagation, a change in the direction of the wave will occur. This changing of direction as the wave
enters the second medium is called REFRACTION. As in the discussion of reflection, the wave striking
the boundary (surface) is called the INCIDENT WAVE, and the imaginary line perpendicular to the
boundary is called the NORMAL. The angle between the incident wave and the normal is called the
ANGLE OF INCIDENCE. As the wave passes through the boundary, it is bent either toward or away
from the normal. The angle between the normal and the path of the wave through the second medium is
the ANGLE OF REFRACTION.
A light wave passing through a block of glass is shown in figure 1-10. The wave moves from point A
to B at a constant speed. This is the incident wave. As the wave penetrates the glass boundary at point B,
the velocity of the wave is slowed down. This causes the wave to bend toward the normal. The wave then
takes the path from point B to C through the glass and becomes BOTH the refracted wave from the top
surface and the incident wave to the lower surface. As the wave passes from the glass to the air (the
second boundary), it is again refracted, this time away from the normal and takes the path from point C to
D. As the wave passes through the last boundary, its velocity increases to the original velocity. As figure
1-10 shows, refracted waves can bend toward or away from the normal. This bending depends on the
velocity of the wave through each medium. The broken line between points B and E is the path that the
wave would travel if the two mediums (air and glass) had the same density.
