1. Dispersion: When light enters a different medium the average speed of light through that medium will change. The speed of a wave is related to the frequency and the wavelength of the wave. If the average speed changes as it enters a new medium will the frequency, wavelength or both change with it? Wavelength - the frequency of a specific color of light is fixed (it is related to the energy the wave is transmitting), therefore the time to complete one cycle is fixed and hence the change in speed would correspond to a change in the distance between two identical points on the wave. When white light (composed of all frequencies of the visible spectrum) enters a medium the average speed of the light decreases. If the speed is related to the wavelength of the light what will happen? The different colors (frequencies) which have different wavelength will now travel at slightly different average speeds. The white light will be separated into its individual colors (frequencies). The separation of white light into its constituent frequencies is called Dispersion. Dispersion – separation of frequencies

2. Dispersion is the principle that governs the formation of a rainbow. Rainbow Formation: Sunlight incident on a raindrop is refracted and disperses the white light into its different frequencies. The curved shape of the back of the raindrop alters the direction of the different frequencies. The light is further refracted as it leaves the raindrop. Only one color (or a small range of colors) will reach your eye from each raindrop. The light from a large number of raindrops allows you to see the full circular arc of a rainbow. A perfect rainbow would be a complete circle. You do not see a circle because there are no raindrops underground. If you were in an airplane you would see a full circle.