When the wavelength of a wave is on the same order of magnitude as the size of a slit, a phenomenon called diffraction occurs. Diffraction refers to the bending or spreading out of a wave as it encounters an obstacle or passes through an opening.
In the context of a slit, such as in the case of light passing through a narrow slit, when the wavelength of the light is comparable to the size of the slit, the diffracted wave spreads out and exhibits a noticeable pattern of interference.
Specifically, if the wavelength of the light is similar to the size of the slit, the diffracted light waves interfere constructively and destructively with each other, resulting in a characteristic diffraction pattern on a screen placed behind the slit.
When the wavelength is similar to the size of the slit, the central part of the diffraction pattern, known as the central maximum or central fringe, is relatively bright and broader compared to the fringes on the sides. The secondary fringes on either side of the central maximum become less distinct and eventually fade away as the angle from the center increases.
This phenomenon is a consequence of the wave nature of light and is described by the principles of wave optics. It is observed not only with light passing through slits but also with other types of waves, such as water waves or sound waves, encountering obstacles or openings of comparable sizes.
It's worth noting that the precise characteristics of the diffraction pattern depend on various factors, including the exact size and shape of the slit, the wavelength of the wave, and the distance between the slit and the screen. These factors collectively determine the specific pattern observed in a given diffraction setup.