Illustration 38.1: Single Slit Diffraction

Please wait for the animation to completely load.

This applet calculates seven frames and then runs continuously. For a large number of sources, or for very small wavelengths, this calculation can take some time, so let the applet finish calculating all seven frames.

To model diffraction from a single slit, we can think of the light entering the slit as point sources for the light exiting (this is effectively Huygen's principle; see Illustration 34.3). The light from these point sources interfere with each other, and diffraction is due to this interference. Restart.

So, in the animation small slit you see five point sources generating the light passing through the slit and the interference pattern from the point sources (diffraction is due to the interference of the waves). Notice that the waves spread out from the slit and that the width of the light (waves) leaving the slit is wider than the slit. It looks as if light "bends" around the corner. Without diffraction, the light exiting the slit would be of the same width as the slit itself.

  • Now look at light passing through a slightly wider slit. What is the difference in the effect of the small slit and the wider slit on the light passing through?
  • If you change the wavelength of the source, the diffraction pattern also changes. Look at a source with a longer wavelength (represented by red color). Then observe a shorter wavelength (represented by blue color). What is the effect of the longer wavelength on the width of the light leaving the slit? In diffraction, as waves pass through a slit, the size of the slit and the wavelength determine how much the waves appear to "bend" around the slit.

For an example of the effect of wavelength and slit size on diffraction, think of the door to your room as a slit. If the wavelength is much smaller than the slit (visible light passing through a door, for example), there is no noticeable diffraction (you see a straight shadow of the door frame). But if the wavelength is much larger than the slit (like a sound wave), there is noticeable diffraction (sound from down the hall bends into your room. It also reflects into your room, so it is hard to separate the effects of diffraction from reflection.). If the wavelength of light were the size of the door, you would see a fuzzy shadow of the door frame.

Illustration authored by Anne J. Cox.

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