Diffraction and Double-Slit Interference

Diffraction is the spreading out of waves as they pass through an opening or travel around an obstacle. When a wave meets a gap or edge, it does not continue in a perfectly straight line but bends into the region behind the barrier. This is why you can hear someone talking around a corner even when you cannot see them, and why water waves spread out after passing through a harbor entrance. Diffraction is a general property of all waves and is direct evidence of their wave nature.

The amount of diffraction depends on the relationship between the wavelength and the size of the opening or obstacle. When the gap is much larger than the wavelength, the wave passes through with little spreading and continues nearly straight. When the gap is comparable to the wavelength, the wave spreads out dram