An optical illusion
Real image perceived as object
31.5 - The law of reflection
Law of reflection: The angle
of incidence equals the angle
of reflection.
At the right, you see a planar mirror. A light ray from
the penguin’s foot is reflecting off the mirror. The light
ray before reflection is called the incident ray; the light
ray that reflects off the mirror is known as the
reflected ray. In this illustration, the rays are in the
same vertical plane.
The angle of incidenceși is shown in the diagrams to the right. The angle și represents
the angle between a line perpendicular (normal) to the mirror’s surface and the incident
ray of light.
The light leaves the surface at the angle of reflectionșr. This is the angle between the
normal line and the reflected ray. As you can see in the diagrams, the angles of
incidence and reflection are the same. This is the law of reflection: The angle of
incidence equals the angle of reflection. This law is confirmed by experiments and
theory.
The law of reflection applies to smooth surfaces, which exhibit specular reflection. The
right-hand ornament in the photograph above is extremely smooth and produces a
specular reflection. Rays that are parallel and close together when they strike the
ornament will all be moving in a new direction after they reflect, but they will still be
parallel. Specular reflection is the topic of this chapter.
If the reflecting surface is rough, diffuse reflection results. The left-hand ornament
above provides an example of diffuse reflection. With a rough surface, neighboring
incident rays will reflect in a variety of directions. Rays that are parallel and close
together when they strike the surface will not be parallel after they reflect. Diffuse
reflection is sometimes desirable. For instance, matte (low gloss) wall paint is designed
to achieve diffuse reflection for surfaces where a shiny appearance is undesirable.
Diffuse and specular reflections from holiday ornaments.The law of reflection
Incidence angle equals reflection angle
Angles measured between light rays
and normal line