Diffraction

Diffraction is the spreading of a beam of waves outside the boundaries set by straight line motion.

To extent to which diffraction occurs depends on the scale of the apparatus compared with the wavelengths involved. A typical Young's two-slits arrangement is about a meter long; about ten million times the wavelength of light. The slits themselves are about a thousand wavelengths across. Diffraction effects are observable on this scale, but only with care. The wavelengths of sound range from a few millimeters to a few meters. On a human scale of things diffraction of sound is very marked. The diffraction of sound through a doorway is more like the diffraction of light through a diffraction grating slit. The diffraction grating slit is typically thinner than the wavelength of light, diffraction is extreme the principle of superposition is needed to decide which directions of light will take.

Curiously, diffraction is not an issue with sound largely because its effect is so dominant; sound goes everywhere. With light, on the other hand, diffraction effects are barely noticeable but they impose a limit on the power of optical instruments to magnify without loss of detail. In optical instruments, diffraction is a vital consideration.