Refraction, Dispersion and Reflection

In nature we are only able to watch the rainbow and enjoy its beauty. The two following pages offer us a possibility with the help of a Java-applet to play actively with refraction, dispersion and with the total reflection of light at the surface between water and air. We will be able to study the principles behind, leading in nature to the appearance of a rainbow.

The first applet demonstrates what happens when a light beam enters or exits a raindrop. In this presentation the boundary surface between air and water is not bend but in order to simplify the understanding.

The position of the yellow marked light source can be changed at will by moving it with the left mouse button. The initial position is in water, which is an optically thicker medium, compared to air. When light beam passes the boundary to the optically thinner medium (air) we see several interesting things.

First we look into reflection: one part of the light beam is reflected back into water. Moving the light source above the limiting angle of 48.6° to the normal, 100% of the incident light is reflected. This is called the total reflection.

Refraction: At smaller angles a part of the light can pass the boundary surface into the other medium. We see that the incoming and the outgoing angles differ. The angle of refraction depends on the color (frequency) of light. Therefore the white light beam from the sun, which is a mixture of light with different frequencies becomes not just refracted, but separated in deflection angle differences into its components. This separation is called dispersion.

The percentage of light that is reflected is shown at the bottom of the applet.

Experiment: Rainbow