Interference, diffraction and seeing them

How interference differs from diffraction has been argued over since both were discovered. Richard Feynman's view in his lectures: there is no important physical difference. When only a few sources, such as two, combine, the result is usually called interference; when very many combine, it is more often called diffraction.

In Young's experiment the pattern on the screen is really double-slit interference superposed on the single-slit diffraction from each slit.

A home demonstration needs two razor blades and a clear glass bulb, ideally with a straight filament. Hold the blades with their edges parallel and a narrow gap between them, keep the gap parallel to the filament, and look through it with the gap right in front of the eye (with spectacles if you wear them).

After adjusting the gap width and keeping the edges parallel, bright and dark bands appear. Apart from the central band, the band positions depend on wavelength, so they show colours. A red or blue filter makes them clearer, and the red bands are visibly wider than the blue ones.

Here the glowing filament stands in for the single source slit S. The eye's lens brings the bands to a focus on the retina, so the retina serves as the screen.

A double slit cut in aluminium foil with a blade repeats Young's experiment with the same bulb. In daylight, the Sun's small bright reflection in a shiny convex surface, such as a cycle bell, can be the source.

Never use the Sun directly: it can damage the eye, and it would give no fringes anyway, because it spans about ½° in the sky, far too wide a source.

Interference and diffraction only redistribute light energy. Wherever a dark fringe has less light, a bright fringe has more; no energy is created or lost, in line with conservation of energy.