1. All waves spread out when they pass through a gap or an obstacle
2. The narrower the gap or the longer the wave, the more the wave spreads out.
3. Narrow gap = a gap almost the same size as the wave
4. Example: light can be diffracted, but as the wave length is small the gap has to be small as well.
1. When a wave crosses the boundary between two substances (e.g glass -> air) it changes direction.
2. Waves are only refracted when they enter the new substance at an angle.