What is dispersion?

Short Answer

Dispersion is the splitting of white light into its different colours when it passes through a medium such as a glass prism. This happens because each colour of light has a different wavelength and therefore bends by a different amount when it enters or leaves the medium.

The result of dispersion can be seen in the rainbow, where sunlight separates into seven colours. Dispersion helps us understand how light behaves, how lenses work, and how optical instruments separate or analyze colours.

Detailed Explanation :

Dispersion

Dispersion is a wave phenomenon in which white light separates into its component colours when it passes through a transparent medium like a prism or water droplets. This separation occurs because different colours of light have different wavelengths, and each wavelength bends by a different amount when it enters or leaves a medium. Dispersion shows that white light is not a single colour but a combination of many colours.

The colours that appear due to dispersion are arranged in a sequence known as VIBGYOR—violet, indigo, blue, green, yellow, orange, and red. Violet has the shortest wavelength and bends the most, while red has the longest wavelength and bends the least.

Why Dispersion Occurs

Dispersion happens due to the dependence of the refractive index on the wavelength of light. When light enters a new medium:

  • Its speed changes
  • Its direction changes (refraction)
  • Its bending differs depending on the wavelength

Shorter wavelengths (violet and blue) slow down more and bend more, while longer wavelengths (orange and red) slow down less and bend less. This difference in bending causes the spreading of colours.

Understanding White Light

White light consists of many colours mixed together. When white light passes through a prism:

  • The prism refracts different wavelengths by different angles
  • These colours fan out into a spectrum
  • The separation becomes visible as rainbow bands

This shows that white light is actually a mixture of many wavelengths.

Role of Refractive Index in Dispersion

The refractive index (n) of a material determines how much light bends when entering it. However, the refractive index is not the same for all wavelengths. This dependence is called chromatic dispersion.

  • Violet light has a higher refractive index in glass
  • Red light has a lower refractive index

This variation causes different wavelengths to separate.

Dispersion in a Glass Prism

When white light enters the prism:

  1. It slows down and bends toward the normal
  2. Different wavelengths bend by different amounts
  3. When leaving the prism, the rays bend again
  4. The total difference in bending creates a spectrum

The angle between the red and violet rays is called the angular dispersion.

Dispersion in Nature

Dispersion explains several natural phenomena:

  1. Rainbow Formation

A rainbow forms when sunlight passes through raindrops, which act like prisms. The light refracts, disperses, reflects inside the droplet, and disperses again when leaving, creating a full spectrum in the sky.

  1. Colorful Patterns in Crystals

Some minerals and crystals show colourful displays due to dispersion.

  1. Halo Around the Sun or Moon

Ice crystals in the atmosphere cause dispersion, forming halos.

Applications of Dispersion

Dispersion has important uses in science and technology:

  1. Spectroscopy

A spectroscope uses dispersion to separate light into a spectrum. Scientists study this spectrum to:

  • Identify chemical elements
  • Study stars and galaxies
  • Analyze gases and materials

Each element produces a unique pattern of colours known as a spectral line.

  1. Optical Instruments

Cameras, telescopes, and microscopes use lenses where dispersion must be controlled to avoid colour distortion.

  1. Fibre Optic Communication

In optical fibres, dispersion affects signal clarity. Engineers design fibres to reduce dispersion so that signals travel long distances without spreading.

  1. Lenses

Dispersion can cause chromatic aberration, where colours focus at different points. Special lenses called achromatic lenses reduce this issue.

  1. Prisms

Prisms are widely used for:

  • Creating rainbows in laboratories
  • Splitting light beams
  • Laser optics
  • Scientific experiments

Types of Dispersion

There are two main types of dispersion:

  1. Material Dispersion

Occurs because refractive index varies with wavelength.

  1. Waveguide Dispersion

Occurs in optical fibres due to different light paths inside the fibre.

Both types affect how signals travel in fibre optics.

Effect of Wavelength on Dispersion

Since dispersion depends on wavelength:

  • Shorter wavelengths bend more
  • Longer wavelengths bend less

This explains why the violet colour appears at the bottom of a prism spectrum and red at the top.

Importance of Dispersion

Dispersion helps scientists and engineers:

  • Understand the composition of light
  • Analyze the light from stars
  • Improve optical devices
  • Create better communication systems
  • Explore natural atmospheric effects

It plays a major role in optics, astronomy, communication, and environmental science.

Conclusion

Dispersion is the splitting of white light into its component colours when it passes through a medium such as a prism or raindrop. It occurs because different wavelengths bend by different amounts. Dispersion explains rainbows, colourful patterns in nature, and the working of optical instruments. It is a key concept in wave optics, helping us understand the true nature of light and its behaviour in different media.