TC 9-64 _________________________________________________________________________
Figure 1-9. Reflection of a Wave
1-38. 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.
Refraction
1-39. 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.
1-40. 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
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