Thin film interference

Bubbles in the air, peacock feathers in bloom and oily puddles all reveal a light phenomenon: thin film interference.

It's a tricky concept involving reflection, refraction and phase changes at interfaces.


Key Concepts

Phase changes at interfaces

When a wave reflects off a loose boundary, it experiences no phase change. However, when a wave reflects off a fixed boundary, it experiences a phase change of \(\pi\) radians.

Conditions for interference

Thin film interference is a result of the combination of the different wavelengths of light and the different distances travelled through the film.

The condition for constructive interference is:

\(2dn=(m+{1\over 2})\lambda\)

  • \(d\) is the thickness of the film (m)
  • \(n\) is the refractive index of the film
  • \(m\) is an integer value
  • \(\lambda\) is the wavelength of light (m)

The condition for destructive interference is:

\(2dn=m\lambda\)

Essentials

Thin film

When solving problems, take care to note what media are surrounding the film. A bubble consists of film between two laters of air.

Oil on a surface has light arriving through air but water underneath.

Wedges of film

Since different wavelengths of light interfere constructively at different path differences, a variation in the thickness of a film will result in the appearance of bands of colour from an original white light source.

This effect can be used to test the flatness of manufactured glass surfaces.

Test Yourself

Use quizzes to practise application of theory.


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