Reflection

A ray diagram shows how light travels, including what happens when it reaches a surface. In a ray diagram, you draw each ray as:

  • a straight line
  • with an arrowhead pointing in the direction that the light travels

Remember to use a ruler and a sharp pencil.

The law of reflection

When light reaches a mirror, it reflects off the surface of the mirror:

A ray diagram showing angle of incidence and angle of reflection.A ray diagram for reflection at a mirror

In the ray diagram:

  • the hatched vertical line on the right represents the mirror
  • the dashed line is called the normal, drawn at 90° to the surface of the mirror
  • the angle of incidence, i, is the angle between the normal and incident ray
  • the angle of reflection, r, is the angle between the normal and reflected ray

The law of reflection states that the angle of incidence equals the angle of reflection, i = r. It works for any angle. For example:

  • the angle of reflection is 30° if the angle of incidence is 30°
  • the angle of reflection is 90° if the angle of incidence is 90°

In the second example, if a light ray travelling along the normal hits a mirror, it is reflected straight back the way it came. The reflection of light from a flat surface such as a mirror is called specular reflection – light meeting the surface in one direction is all reflected in one direction.

Scattering

If light meets a rough surface, each ray obeys the law of reflection. However, the different parts of the rough surface point in different directions, so the light is not all reflected in one direction. Instead, the light is reflected in all directions. This is called diffuse scattering. It explains why you can see a clear image of yourself in a shiny flat mirror, but not in a dull rough wall.