Why do waves spread when passing through a narrow slit?

Waves spread out when passing through a narrow 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 bending and spreading out of waves when they encounter an obstacle or pass through a narrow opening, such as a slit. This phenomenon is most noticeable when the size of the obstacle or opening is comparable to the wavelength of the wave.

The principle behind diffraction can be explained using Huygens' Principle, which states that every point on a wavefront can be considered as a source of secondary wavelets. These secondary wavelets spread out in all directions, and the new wavefront is the envelope of these wavelets. When a wave passes through a narrow slit, the wavefront is confined to the width of the slit. According to Huygens' Principle, each point along this narrow wavefront acts as a source of secondary wavelets, which spread out in all directions. This results in the wave spreading out, or diffracting, as it passes through the slit.

The amount of diffraction, or spreading, depends on the wavelength of the wave and the width of the slit. If the slit is much wider than the wavelength of the wave, there will be little diffraction and the wave will continue in a straight line. However, if the slit is comparable to or smaller than the wavelength of the wave, significant diffraction will occur and the wave will spread out considerably.

This phenomenon is not only observed in physical waves like sound and water waves, but also in light waves. This is the reason why we observe patterns of light and dark bands when light passes through a narrow slit, a phenomenon known as interference. This principle is fundamental in the study of wave optics and has numerous applications in fields such as astronomy, telecommunications, and medical imaging.