Refraction at a boundary

Light travels in a straight line through a uniform transparent medium. When it crosses into a different transparent medium, its speed changes. This can also change the light’s direction. The change in direction at the boundary is called refraction.

For a ray that meets the boundary at an angle:

• If light travels more slowly in the new medium, the ray bends towards the normal.
• If light travels faster in the new medium, the ray bends away from the normal.

For example, when light passes from air into glass, it slows down and bends towards the normal. When it passes from glass into air, it speeds up and bends away from the normal.

The normal and the angles

The normal is an imaginary line perpendicular to the boundary at the point where the ray meets it. It provides the reference line for measuring both angles:

• The angle of incidence is between the incoming, or incident, ray and the normal.
• The angle of refraction is between the refracted ray and the normal.

Both angles are measured from the normal, not from the boundary. Bending towards the normal means the angle of refraction is smaller than the angle of incidence; bending away means it is larger.

A ray along the normal

A ray that meets the boundary along the normal has an angle of incidence of zero. Its speed changes as it enters the new medium, but its direction does not change. It continues straight along the normal.