In Young's double-slit experiment, dark fringes are regions where no light or minimal light intensity is observed on the screen placed behind the double slits. These dark fringes occur due to destructive interference between the two waves originating from the two slits.
When coherent light (light with a single wavelength and constant phase relationship) passes through the double slits, it splits into two secondary wavefronts that emanate from each slit. These secondary wavefronts then propagate and overlap with each other, creating an interference pattern on the screen.
At certain locations on the screen, the path lengths from the two slits to a particular point are such that the waves arrive out of phase with each other. This phase difference leads to destructive interference, where the crests of one wave coincide with the troughs of the other wave, resulting in cancellation of amplitudes and minimal or no light intensity at that specific point on the screen. These regions of destructive interference appear as dark fringes.
The positions of the dark fringes can be determined by using the principles of wave interference, taking into account factors such as the distance between the slits, the wavelength of the light, and the distance between the screen and the double-slit setup.
The presence of dark fringes, along with bright fringes, in the double-slit interference pattern is a characteristic feature of wave interference and serves as evidence of the wave nature of light.