Dispersion is defined as the spreading of white light into its full spectrum of wavelengths.
Refraction is responsible for dispersion in rainbows and many other situations. We know that the index of refraction (n) depends on the medium. For a given medium, n also depends on wavelength: n increases as wavelength decreases and is greatest for violet light. White light, in particular, is a relatively uniform mixture of all visible wavelengths (ROYGBIV). Violet light is refracted more than red light, and the light is dispersed into the same sequence of wavelengths.
A pure wavelength of light falls onto a prism and is refracted at both surfaces. The prism disperses white light.
A combination of refraction and reflection produces rainbows. The light is refracted both as it enters and as it leaves the drop. Since the index of refraction of water varies with wavelength, the light is dispersed, and a rainbow is observed. (There is no dispersion caused by reflection at the back surface since the law of reflection does not depend on wavelength.)
Key Points
• Dispersion occurs whenever there is a process that changes the direction of light in a manner that depends on wavelength. Dispersion can occur for any type of wave and always involves wavelength-dependent processes.
• For a given medium, n increases as wavelength decreases and is greatest for violet light. Thus violet light is bent more than red light, as can be seen with a prism.
• In a rainbow, light enters a drop of water and is reflected from the back of the drop. The light is refracted both as it enters and as it leaves the drop.
Key Terms
index of refraction: for a material, the ratio of the speed of light in a vacuum to that in the material
medium: material that light moves through
white light: the term used to describe the spectrum of light
