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Myths, Mistakes, & Misunderstandings

The Light Saber

The Star Wars light saber is an impossible myth. Assuming that
the light saber is a laser beam, we know that one laser beam
will not stop another laser beam. Light beams simply pass
through each other. Furthermore, a laser with a fixed length is
not possible without a system of lenses that would also scatter
the light, in addition to being cumbersome on a saber. More- FIGURE 7.11 The law of reflection states that the angle of in-
over, scattered laser light from reflective surfaces could injure cidence (θ i ) is equal to the angle of reflection (θ r ). Both angles are
the person holding the saber. measured from the normal, a reference line drawn perpendicular to
the surface at the point of reflection.

in which they are reflected from the mirror to reach your eyes

REFLECTION
can be understood by drawing three lines: (1) a line represent-
Most of the objects that you see are visible from diff use refl ection. ing an original ray from the tree, called the incident ray, (2) a
For example, consider some object such as a tree that you see dur- line representing a reflected ray, called the refl ected ray, and

ing a bright day. Each point on the tree must reflect light in all (3) a reference line that is perpendicular to the refl ecting surface

directions, since you can see any part of the tree from any angle and is located at the point where the incident ray struck the sur-
(Figure 7.10). As a model, think of bundles of light rays entering face. This line is called the normal. The angle between the inci-

your eye, which enable you to see the tree. This means that you can dent ray and the normal is called the angle of incidence, θ i , and
see any part of the tree from any angle because diff erent bundles the angle between the reflected ray and the normal is called the

of reflected rays will enter your eye from different parts of the tree. angle of refl ection, θ r (Figure 7.11). Th e law of refl ection, which

Light rays that are diff usely reflected move in all possible was known to the ancient Greeks, is that the angle of incidence

directions, but rays that are reflected from a smooth surface, equals the angle of refl ection, or

such as a mirror, leave the mirror in a definite direction. Sup-

pose you look at a tree in a mirror. There is only one place on θ i = θ r
the mirror where you look to see any one part of the tree. Light equation 7.1

is refl ecting off the mirror from all parts of the tree, but the

Figure 7.12 shows how the law of reflection works when
only rays that reach your eyes are the rays that are refl ected at

you look at a flat mirror. Light is reflected from all points on
a certain angle from the place where you look. Th e relationship
the block, and of course only the rays that reach your eyes are
between the light rays moving from the tree and the direction

detected. These rays are refl ected according to the law of refl ec-

tion, with the angle of reflection equaling the angle of incidence.
If you move your head slightly, then a different bundle of rays

reaches your eyes. Of all the bundles of rays that reach your eyes,
only two rays from a point are shown in the illustration. Aft er
these two rays are reflected, they continue to spread apart at the

FIGURE 7.12 Light rays leaving a point on the block are
FIGURE 7.10 Bundles of light rays are reflected diffusely in reflected according to the law of reflection, and those reaching your
all directions from every point on an object. Only a few light rays eye are seen. After reflecting, the rays continue to spread apart at
are shown from only one point on the tree in this illustration. The the same rate. You interpret this to be a block the same distance
light rays that move to your eyes enable you to see the particular behind the mirror. You see a virtual image of the block, because
point from which they were reflected. light rays do not actually move from the image.

182 CHAPTER 7 Light 7-6