Interference and Diffraction of Light

Evidence that light behaves as a wave.

When two light waves overlap they add. Where crests meet crests the light is bright, and where crests meet troughs it cancels to dark. In Young's double-slit experiment this produces evenly spaced bright and dark fringes.

Diffraction is the spreading of waves around edges and through narrow gaps. A single slit gives a wide bright central band with weaker bands on either side. Both effects are only noticeable when the gap is comparable to the wavelength.

• Constructive interference: path difference is a whole number of wavelengths.
• Destructive interference: an odd number of half wavelengths.
• Narrower slits spread the light more.

Browse all 50 physics and electronics pages  |  Back to the course index