Reflection of Light in Optics

Specular and Diffuse; Equality of Incident and Reflected Angles

Apr 15, 2008

When light is reflected the incident and reflected angles are equal. Specular reflection is from a smooth surface, and diffuse reflection from a rough surface.

Law of Reflection

When a pool player bounces a pool ball off the wall of a pool table, it bounces, or reflects, off the wall in a manner similar to light reflecting off a mirror. Those experienced at pool know that, if not spinning, the pool ball will bounce off the wall at the same angle it strikes the wall.

The law of reflection for light says that light will do the same thing. The angle at which the light strikes the mirror, or other reflective surface, is called the incident angle. The angle of the reflected light ray is called the reflected angle. The law of reflection simply states that the incident angle equals the reflected angle. In addition the incident rays, reflected rays, and the perpendicular to the surface are all in the same plane.

There is however one minor source of possible confusion. Physicists do not measure these angles from the reflecting surface. Rather physicists measure the angles from a line that is drawn perpendicular to the reflecting surface, which is called the normal. In mathematics the word normal means perpendicular.

This type of reflection from a mirror or other polished reflective surface is called specular reflection. The word specular derives from the Latin word for mirror, speculum.

Diffuse Reflection

For specular reflection to work, the surface must be very smooth. Reflection from a rough surface is called diffuse reflection. The reflected light is diffuse and does not produce a mirror image.

Diffuse reflection may not appear to follow the law of reflection, however it does when examined at a microscopic level. If the reflecting surface is rough, the rays of light incident on different points of the surface appear to be reflected at different angles. However at the microscopic level each individual light ray follows the law of reflection for the precise point on the surface it strikes.

Because the surface is rough with many microscopic hills and valleys each point on the surface is oriented at a different angle. Individual light rays have equal incident and reflected angles for the points they strike, but a large number of rays will reflect at many different angles. Hence the reflected light from a rough surface is diffuse and appears to be reflected at many different angles.

Reflection off a light colored wall is a good example of diffuse reflection. The light reflects back to the room, but it is not possible to see mirror images.

Reflection from a pond or lake is a nice example combining both specular and diffuse reflection. Where the water is very still, the surface is smooth. It is possible to see a reflected image because this still water produces specular reflection. Where the water is less still, with perhaps a few gentle waves, the reflection is diffuse. The water appears bright, but it is not possible to see a reflected image.