1-99. Prior to the mid-seventeenth century, scientists thought that light
required no time at all to pass from the source to the observer. Then, in 1675,
Ole Roemer, a Danish astronomer, discovered that light travels
approximately 186,000 miles per second in space. At this velocity, a light
beam can circle the earth 7 1/2 times in one second.
1-100. The speed of light depends on the medium through which the light
travels. In empty space, the speed is 186,000 (1.86 x 105) miles per second. It
is almost the same in air. In water, it slows down to approximately 140,000
(1.4 x 105) miles per second. In glass, the speed of light is 124,000 (1.24 x 105)
miles per second. In other words, the speed of light decreases as the density
of the substance through which the light passes increases.
1-101. The velocity of light, which is the same as the velocity of other
electromagnetic waves, is considered to be constant, at 186,000 miles per
second. If expressed in meters, the velocity of light is 300,000,000 meters per
1-102. Light waves obey the law of reflection in the same manner as other
types of waves. Consider the straight path of a light ray admitted through a
narrow slit into a darkened room. The straight path of the beam is made
visible by illuminated dust particles suspended in the air. If the light beam is
made to fall onto the surface of a mirror or other reflecting surface, however,
the direction of the beam changes sharply. The light can be reflected in
almost any direction, depending on the angle at which the mirror is held.
1-103. As shown earlier in figure 1-9, if a light beam strikes a mirror, the
angle at which the beam is reflected depends on the angle at which it strikes
the mirror. The beam approaching the mirror is the incident or striking
beam, and the beam leaving the mirror is the reflected beam.
1-104. The term "reflected light" simply refers to light waves that are
neither transmitted nor absorbed, but are thrown back from the surface of
the medium they encounter. This phenomenon is described more fully in the
discussions of radio waves (chapter 2) and antennas (chapter 4).
Refraction of Light
1-105. The change of direction that occurs when a ray of light passes from
one transparent substance into another of different density is called
refraction. Refraction is due to the fact that light travels at various speeds in
different transparent substances. For example, water never appears as deep
as it really is, and objects under water appear to be closer to the surface than
they really are. A bending of the light rays causes these impressions.
1-106. Another example of refraction is the apparent bending of a spoon
when it is immersed in a cup of water. The bending seems to take place at the
surface of the water, or exactly at the point where there is a change of
density. Obviously, the spoon does not bend from the pressure of the water.
The light forming the image of the spoon is bent as it passes from the water
(a medium of high density) to the air (a medium of comparatively low