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Dispersion is the phenomenon where white light is separated into its constituent colours (the spectrum) when it passes through a prism. This occurs because the refractive index of glass is slightly different for different wavelengths of light. This difference in refractive index causes different colours to bend by different amounts, resulting in separation.
A prism is a triangular-shaped piece of glass. When white light enters the prism, it is refracted (bent) at the first surface. Because the refractive index of glass varies with wavelength, different colours are refracted at slightly different angles. This initial separation of colours is then further separated as the light exits the prism at the second surface.
White light is a combination of all the colours of the visible spectrum. Each colour has a different wavelength. The refractive index of the prism material (glass) is not the same for all wavelengths. Shorter wavelengths (e.g., violet and blue) are refracted more than longer wavelengths (e.g., red). This difference in refraction is what causes dispersion.
The angle of deviation (the amount the light is bent) is different for each colour. Violet light is deviated the most, and red light is deviated the least.
The refractive index ($n$) of a material is related to the wavelength ($\lambda$) of light by the following approximate equation:
$$n = \frac{c}{\lambda}$$
where $c$ is the speed of light in a vacuum.
Since the speed of light in a vacuum is constant, this equation shows that the refractive index of a material is inversely proportional to the wavelength of light. This means that shorter wavelengths experience a higher refractive index and are slowed down more than longer wavelengths.
The spectrum produced by a prism typically shows the colours in the following order (from the top, where the light enters the prism, to the bottom, where it exits):
Dispersion is used in various optical instruments, such as prisms in spectrometers and spectroscopes, which are used to analyse the composition of light. By examining the spectrum of light from a star, for example, astronomers can determine its chemical composition.
| Colour | Approximate Wavelength (nm) | Approximate Refractive Index |
|---|---|---|
| Violet | 380-450 | 1.52 |
| Indigo | 450-480 | 1.52 |
| Blue | 480-510 | 1.52 |
| Green | 510-570 | 1.52 |
| Yellow | 570-590 | 1.52 |
| Orange | 590-620 | 1.52 |
| Red | 620-750 | 1.52 |