1-30
Q36. White light falls upon a dull, rough, dark-brown object. Will the light primarily be reflected,
diffused, or absorbed by the object?
Q37. What color will be emitted by a dull, rough, black object when white light falls upon it?
Q38. A substance that transmits light but through which an object cannot be seen clearly is known as
what kind of substance?
Speed of Light
You probably have heard people say, "quick as lightning" or "fast as light" to describe rapid motion;
nevertheless, it is difficult to realize how fast light actually travels. Not until recent years have scientists
been able to measure accurately the speed of light.
Prior to the middle 17th 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.
The speed of light depends on the medium through which the light travels. In empty space, the speed
is 186,000 (1.86 105) miles per second. It is almost the same in air. In water, it slows down to
approximately 140,000 (1.4 105) miles per second. In glass, the speed of light is 124,000 (1.24 10
5)
miles per second. In other words, the speed of light decreases as the density of the substance through
which the light passes increases.
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, it is 300,000,000 meters per second.
Reflection of Light
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.
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.
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.
You will see this application used in our discussion of radio waves (chapter 2) and antennas (chapter
4).
Q39. At what speed does light travel?
Refraction of Light
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
