Diffraction and interference
Waves bend around edges and spread through gaps, then overlap to make patterns of strong and weak, bright and dark.
Diffraction is the way a wave spreads out when it passes an edge or squeezes through a gap. It happens to every kind of wave: water, sound and light. The effect is strongest when the gap or obstacle is about the same size as the wavelength, which is why you can hear someone talking round a corner (sound waves are tens of centimetres to metres long) but cannot see them (light waves are less than a thousandth of a millimetre).
Diffraction goes hand in hand with interference. Where spreading waves overlap, crest meeting crest makes a bigger wave (constructive interference) and crest meeting trough cancels out (destructive interference). The result is a pattern of strong and weak bands, like the criss-cross ripples behind a harbour entrance or the rainbow sheen on a CD.
One of the great tests of the wave theory of light came from a competition run by the French Academy of Sciences in 1818. Augustin Fresnel entered a wave theory of diffraction. Siméon-Denis Poisson, a supporter of Newton's view that light is made of particles, pointed out that it predicted something that seemed absurd: a bright spot at the very centre of the circular shadow of a round object. François Arago did the experiment and found the spot. It is now called Arago's or Poisson's spot.
Coming up in future stories
- The Sailors Who Mapped Waves With Sticks 19th century · Marshall Islands
- The Spot That Was Supposed to Be Impossible 1818 · Paris, France