Reflection off smooth surfaces such as mirrors is known as specular reflection. Reflection off rough surfaces such as clothing or paper is known as diffuse reflection (Figure 2.16).
Figure 2.16: Specular and diffuse reflection
With diffuse reflection light is reflected in all directions.
The angles i and r are measured from the normal.
With specular reflection all the reflected rays are parallel to each other.
When light is reflected off a plane mirror, the angle of incidence (i) is equal to the angle of reflection (r) (Figure 2.17). This is called the law of reflection. The dashed line is called the normal. It is at 90° to the plane mirror and is the line from which all angles are measured. The incident ray (i) is the ray coming in from the light source to the mirror. The reflected ray (r) is the ray coming away from the surface of the mirror.
Figure 2.17: Light reflecting off a plane mirror
green1.png A ray of light hits a plane (flat) mirror at an angle of incidence of 40°.
  1. What type of reflection is this?
  2. How big is the angle of reflection?
green2.png The angle between a plane mirror and a ray of light is 35°. How big is the angle of reflection?