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Shine light through two narrow, closely-spaced slits and, instead of two bright bands, you get a whole pattern of alternating bright and dark fringes. This single experiment — Young’s double slit — was one of the most important results in the history of physics, providing direct proof that light behaves as a wave.
What you'll be able to do
A stable, observable interference pattern requires two sources — sources of the same frequency and wavelength, with a constant phase difference between them. is the difference in distance travelled by waves from two sources to a given point; constructive interference (a bright fringe) occurs where path difference is a whole number of wavelengths, and destructive interference (a dark fringe) occurs where it is a whole number plus a half wavelength.
Light passing through two narrow, closely-spaced slits diffracts at each slit and the two resulting waves overlap, producing a pattern of alternating bright and dark fringes on a screen — direct evidence for the wave nature of light. The spacing between adjacent bright fringes is linked to the wavelength, the slit separation, and the distance to the screen.
Tip — Always convert every length to metres before substituting into the fringe equation — mixing mm and m is one of the most common sources of an answer wrong by a power of ten.
Equation recap
Common mistakes to avoid
Key takeaways
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