Why do waves spread out after passing through a slit?

Waves spread out after passing through a slit due to a phenomenon known as diffraction.

Diffraction is a fundamental characteristic of waves, including light, sound, and water waves. It refers to the spreading out of waves when they encounter an obstacle or pass through a gap, such as a slit. The size of the slit relative to the wavelength of the wave determines the extent of the diffraction. If the slit is much larger than the wavelength, the wave will not spread out much. However, if the slit is comparable to or smaller than the wavelength, the wave will spread out significantly.

The principle behind this phenomenon is Huygens' Principle, which states that every point on a wavefront can be considered as a source of secondary wavelets that spread out in all directions with the same speed as the wave itself. When a wave passes through a slit, the wavefront is effectively reduced to the width of the slit. The points along this narrow wavefront then act as sources of new waves, spreading out in a semi-circular or semi-spherical pattern, depending on whether the wave is two-dimensional or three-dimensional.

The pattern of the diffracted wave can be predicted using the principle of superposition, which states that the total displacement of the wave at any point is the sum of the displacements of the individual waves. This can result in constructive or destructive interference, creating a pattern of bright and dark bands in the case of light waves, or loud and quiet regions in the case of sound waves.

Understanding diffraction is crucial in many areas of physics and engineering, including the design of optical and audio equipment, the study of wave behaviour in nature, and even the detection of distant planets in astronomy. It's a fascinating demonstration of the wave nature of light and sound, and a key part of the IB Physics syllabus.