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Interference and Diffraction of Light Revision Notes

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12.4Interference and Diffraction of Light


In these revision notes for Interference and Diffraction of Light, we cover the following key points:

  • Is there any diffraction in light waves?
  • Why diffraction and interference of light are related phenomena?
  • How to obtain light diffraction on a screen using a single slit?
  • What Young Double-Slit Experiment is used for?
  • Why diffraction gratings are used in technology?
  • How to find wavelength of a light wave using diffraction gratings?

Interference and Diffraction of Light Revision Notes

Light is an electromagnetic transverse wave. As such, it possesses all properties of waves, including interference and diffraction. Since diffraction is observed only when the dimensions of the obstacle or gap are comparable to the wavelength, the diffraction of light waves can hardly be observed without using special techniques due to the very small wavelengths of light waves.

When light passes through a single slit whose width a is on the order of the wavelength of the light, then we can observe a single slit diffraction pattern on a screen that is a distance D >> a away from the slit. The light intensity is a function of angle, i.e the intensity of light weakens when the angle increases.

This known equation

d2-d1 = a × sinθ

helps us determine the wavelength λ if we know the slit's width a and the angle θ formed by the rays to the horizontal direction, because we have

d2-d1=a × sinθ = N × λ

for constructive interference and

d2-d1=a × sinθ = 2N - 1/2 × λ

for destructive interference.

The famous scientist Thomas Young developed a technique that is similar to the one described above to calculate the wavelength of a light wave. The only difference is that Young used two slits instead of a single slit to produce interference on the screen through diffraction of light waves.

To determine the wavelength λ of the two coherent light waves passing through the slits, we must know three quantities:

  1. The distance (separation) between the two slits, a
  2. The distance (separation) between two fringes, i.e. the distance between the centres of two adjacent bright (or dark) regions, x and
  3. The distance from the slits to the screen, D.

Thus, the wavelength λ is calculated by the equation

λ = a × x/D

A diffraction grating tool is very similar in concept to the double-slit system discussed earlier but with more openings. It has a very large number of parallel lines on a glass or plastics obtained through grating process.

A diffraction grating is often (but not only) used to split the white light into colours. It is also used to determine the wavelength of a light wave similarly to the methods mentioned earlier in single-slit diffraction and Young Double-Slit Experiment.

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