Science Puzzle
Can Sound Turn a Corner but Light Cannot?
Stand round the corner from an open doorway and you can hear a conversation inside perfectly clearly, even though you cannot see a single thing in the room. Sound reaches you; light does not.
Both sound and light are waves. Why does only one of them bend around the corner?
The Answer
Both bend. Sound just bends far more, and the reason is the size of the wave compared with the size of the doorway.
When a wave squeezes through an opening, it spreads out on the far side. How much it spreads depends on how the wave's length compares with the width of the gap. Sound waves in speech are around a metre long, comparable to a doorway, so they fan out dramatically and flood the corridor beyond. Light waves are less than a thousandth of a millimetre long, millions of times smaller than the door, so they barely notice the opening at all and march straight on, leaving a sharp-edged shadow.
Light does bend around edges, just by an amount far too small to see without careful equipment. Shine a laser through a hair-thin slit and you will watch it fan out into stripes, behaving exactly like sound in a doorway.
The principle: Diffraction. Waves spread out when passing an obstacle or opening, and the spreading is large only when the wavelength is comparable to the size of the gap.