ABSTRACT
Here’s one you can do for yourself in the garden shed. Point a large, powerful telescope at a bright star and, instead of looking through it, project the single point of starlight onto a piece of card. Rather than a bright dot of light, you will see a bright dot of light surrounded by a bit of blurring. The light has spread out, by the phenomenon known as diffraction, the process seen most easily with waves in liquids. If a wave goes through a 10-metre gap in a sea wall, it does not carry on as a 10-metre-wide wave, in an area of calm water, but immediately spreads out. The amount of diffraction with light is very small, and we can normally suppose light to be little parcels of energy that travel in straight lines. But they don’t always.
