Interference of Light

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Light waves from two coherent sources interfere. In Young's double-slit experiment, light passing through two narrow slits forms alternating bright and dark fringes on a screen, with spacing w = λD/d. Bright fringes occur where path difference is a whole number of wavelengths.

Here d is the slit separation and D is the distance to the screen. The experiment in 1801 gave strong evidence that light is a wave, since particles would simply make two bright strips. Using white light gives coloured fringes since each wavelength has a different spacing, with red spreading wider than blue. Dark fringes appear where the path difference is an odd number of half wavelengths.

Worked example: green light of 500 nm through slits 0.5 mm apart on a screen 2 m away gives fringe spacing w = 500 × 10⁻⁹ × 2 / (0.5 × 10⁻³) = 0.002 m, 2 mm. That is easy to see, so the experiment can measure the wavelength of light with a ruler.

Thin-film interference gives the colours on soap bubbles and oil slicks, and anti-reflection coatings on lenses use destructive interference to reduce glare. Interferometers measure tiny distances, and LIGO used them to detect gravitational waves. A misconception is that the dark fringes lose energy; it is redirected to the bright ones. Two ordinary bulbs show no pattern because they are not coherent.