Figure 16.7 illustrates a ray of light being reflected
from a plane mirror. We can summarize this behavior in the law
of reflection, that the angle of reflection equals the angle of
incidence. The angle of reflection describes the direction of
the outgoing light, after it has been reflected from the mirror.
It is customary to measure this angle from the normal, the direction
perpendicular to the mirror, as shown in the figure. The angle
of incidence is the angle of the incoming or incident light, also
measured from the direction perpendicular to the mirror. In the
figure, this means the horizontal motion of the light before and
after reflection is unchanged while the vertical motion is just
reversed by the reflection. This behavior of light is similar
to that of a tennis ball being bounced against a wall or the floor.
Figure 16.7 For reflection from a plane mirror-or
other, similar smooth or polished surface-light leaves the surface
with the same angle as it arrived. We describe this by saying
the angle of reflection equals the angle of incidence, reflection
= incidence.
Figure 16.8 A wide beam of light reflects from
a mirror much like a thin beam or ray of light. It is easy to
see the law of reflection in operation for this reflection.
A wide beam of light reflects from a mirror just like a thin beam
or ray of light. It is easy to see the law of reflection in operation
for this reflection, as illustrated in Figure 16.8; this is called
specular reflection and occurs from very smooth surfaces such
as metal, glass, or water. While paper feels smooth to the touch
of a finger, it is really quite irregular or rough on a microscopic
scale. If a wide beam of light shines on a piece of paper-or most
other seemingly smooth surfaces-the law of reflection still holds
for each individual ray of light that makes up the wider beam.
However, the surface of paper is quite irregular so the angles
of incidence vary greatly from one point to another nearby point.
That means the angles of reflection also vary greatly. That means
the light is reflected-or scattered-into many different directions.
This type of reflection is called diffuse reflection and is illustrated
in Figure 16.9.
Figure 16.9 Each individual ray of light still
obeys the law of reflection that the angle of reflection equals
the angle of incidence. But for reflection from a dull or rough
surface, the surface itself varies so much that light is reflected-or
scattered-in all directions.
Q: Why is there a mirror atop an overhead projector?
A: The mirror reflects the light, changing its direction.
Without the mirror, the lens of an overhead projector would form
an image on the ceiling. The mirror reflects the light, changing
its direction so an image is formed on a screen. By adjusting
the mirror, the angles can be changed so the image may be moved
up or down.
