Young’s double slit interference of light

Predictions of the particle and wave models of light for Young's double slit experiment
In 1801, the English physicist Thomas Young devised an experiment to determine whether light behaved as a wave or a particle. Up until then, physicists disagreed on the nature of light. Young’s insight was to pass monochromatic and coherent light through two narrow and closely spaced slits and then observe the pattern of the light on a screen. (Coherent light means that the waves are in phase with each other.) If light were composed of particles, then the particles would pass straight through the two slits and produce two bright lines of light on the screen. If light acted as a wave, then a distinctive interference pattern would be created by light waves passing through the slits.
Read the text aloud Show Light source for Young’s experiment
Young found that light interfered with itself to produce a set of interference fringes on the screen. The brightest fringe was located between the slits, a location predicted by the wave model but where no particle could have reached. Interference is a behavior of waves, so Young’s experiment was the first direct confirmation that light can act as a wave. Young’s results were so convincing that 19th-century physicists assumed that light was a wave and thoroughly rejected the particle model until Albert Einstein proposed a theory that successfully explained the photoelectric effect. Read the text aloud
Young's double slit experiment that showed light exhibits wave behavior by interfering with itself
What causes the interference pattern? Light through both slits will travel an equal distance to a location on the screen halfway between the two slits, producing a central maximum. At a small angle θ from the center, light from one slit will have to travel an additional distance of λ/2, or one-half of a wavelength. That causes destructive interference, because the two waves are out of phase. At a larger angle, light from one slit will have to travel an additional distance of λ, or one full wavelength. That causes constructive interference, because the two waves are fully in phase. The pattern repeats as the angle increases, resulting in an interference pattern. Read the text aloud Show Equations for the location of Young’s fringes

Previous Page Next Page651