Unit 2 Section 3 Diffraction


The Diffraction of Light - Explanation

  • Light shone through a narrow slit will diffract and sometimes produce a diffraction pattern.
  • To observe a light diffraction pattern, you need to use a light source that is monochromatic
  • Monochromatic: A light source that is all of the same wavelength (or frequency).
  • If non-monochromatic light is used, then the different wavelengths will diffract by different amounts and the pattern produced won't be very clear.


  • White light is made up of a continous range of wavelengths across the visible light spectrum.
  • You can put a colour filter in front of white light to make it a single wavelength, but using a laser gives clearer diffraction patterns
  • Lasers are monochromatic but also dangerous enough that they could damage your eyesight if looked at directly.
  • The diffraction of light is shown by shining a laser beam through a very narrow slit onto a screen.
  • You can alter the amount of diffraction by changing the width of the slit.
  • If the wavelength of a light wave is roughly similar to the size of the slit, you get a diffraction pattern of light and dark fringes.
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The Diffraction of Light - Diagram

  • Light fringes are due to constructive interference, where waves are in phase.
  • Dark fringes are due to total destructive interference, where waves are completely out of phase.


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Diffraction through Gaps

The amount of diffraction depends on the wavelength of the wave compared with the size of the gap:

  • When the gap is much bigger than the wavelength, diffraction is unnoticeable.
  • You get noticeable diffraction through a gap several wavelengths wide.
  • The most diffraction is when the gap is the same size as the wavelength
  • If the gap is smaller than the wavelength, the waves are mostly reflected back.


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