May 15 2014
Dispersion is the process of describing the characteristics of light waves and their interaction with a medium, so that the group and phase velocities of the light propagating the medium depend on the optical frequency.
Dispersion of a material is measured by its Abbe number, V. Strong dispersion will have low Abbe numbers. The most common example of dispersion is a rainbow, in which a white light is separated into components of different wavelengths.
Basic Principle
When a light ray passes from one medium to another, it gets refracted and its speed changes. The light is bent at the interface and enters a dense material in one direction, if its velocity slows down, and a less dense material in another direction, if the light velocity speeds up.
However, the amount of bending varies from one wavelength to another. Light with a short wavelength travels more slowly via the medium than light with a long wavelength.
Violet has a short wavelength and, hence, it is bent the most. As white light is comprised of different visible wavelengths, the colors are dispersed due to this difference in behavior.
Light passing through a glass encounters two interfaces – one while entering, and the other during exit. The bending and dispersion taking place at the first interface are exactly reversed at the second interface, if the two interface surfaces are parallel to each other.
Thus, the emerging light ray travels in the direction of incoming light and is slightly dispersed from it. Also, the wavelengths dispersed at the first interface are recombined at the second interface.
Applications
Some of the major applications of optical dispersion are as follows:
- Laser tuning
- Measurements using spectroscopy
- Optical coherence tomography systems
References