# What is the optical density of light

## Refraction

#### Rays of light at the transition from air to water

A ray of light spreads in a straight line through the air. If the light beam hits a water surface, it is split up. Part of the light beam continues to propagate in the water and another part of the light beam is reflected back into the air.

The part of the light beam that propagates further in the water changes its direction of propagation when it passes from air to water. This is called refraction. You can use the angle to measure how strongly the light beam is refracted. In Figure 1 you can see that the angle of incidence \ ({\ alpha_ {1}} \) is larger than the angle of refraction \ ({\ alpha_ {2}} \). The refracted light beam then spreads out in a straight line again in the water.

The other part of the light beam is reflected at the transition from air to water. You can also measure the angles here. According to the law of reflection, the angle of incidence \ ({\ alpha_ {1}} \) in Figure 1 is exactly as large as the angle of reflection \ ({\ alpha ^ {'} _ {1}} \).
In the air, the reflected light beam then spreads out in a straight line again.

#### Rays of light at any boundary

A ray of light usually always changes its direction of propagation at the transition between two media, not only at the transition from air to water. If a light beam from a transparent medium 1 enters a likewise transparent medium 2, the light beam changes its direction of propagation. The light beam is refracted. The angle \ ({\ alpha_ {1}} \) between the incident light beam and the perpendicular to the interface is not equal to the angle \ ({\ alpha_ {2}} \) between the refracted light beam and the perpendicular. Part of the light beam is reflected at the interface between the media \ ({\ left (\ alpha_ {1} = \ alpha ^ {'} _ {1} \ right)} \).

Incident light beam, plumb line, refracted light beam and reflected light beam lie in one plane. This is called the plane of incidence.