If you look at the surface of a lake, you can see sunlight reflecting off the water. But you also see rocks and plants
under the surface. Because you see them, light must have entered the water, reflected off the rocks and plants, returned to the water surface, and then traveled from the surface to your eyes. When you look carefully at objects in the water, however, you notice that they look closer than normal. This phenomenon occurs because light is bent at the boundary between the water and the air around it. The bending of light as it travels from one medium to another is called refraction. In this section you will learn how and when refraction occurs including the laws of refraction.

Key Terms

• Refraction
• Medium
• Speed of light

Refraction

When light crosses the boundary between two transparent materials, some of it is reflected and some is transmitted. For example, when a light ray traveling in air is incident on a transparent material such as glass, it is partially reflected and partially transmitted (Figure 1). But the direction of the transmitted light is different from the direction of the incident light, so the light is said to have been refracted; in other words, it has changed direction.

Refraction refers to the change in direction of a light ray at a boundary where it passes from one medium to another.

As in the case of reflection, the angles of the incident and refracted rays are measured with respect to the normal. For studying refraction, the normal line is extended into the refracting medium, as shown in Figure 1. The angle between the refracted ray and the normal is called the angle of refraction, θ₂, and the angle of incidence is designated as θ₁.