In Young's double-slit experiment, the observation of dark and bright bands is a result of the interference of light waves. When light passes through two closely spaced slits, it diffracts and creates a pattern of alternating bright and dark regions on a screen placed behind the slits.
This phenomenon can be explained by considering the wave nature of light. Each slit acts as a source of spherical waves that spread out and overlap with each other. Where the peaks of the waves coincide, constructive interference occurs, resulting in bright regions. Conversely, where the peaks and troughs of the waves coincide, destructive interference occurs, leading to dark regions.
The bright bands correspond to points where the path difference between the waves from the two slits is an integer multiple of the wavelength of light. In these regions, the waves arrive in phase and reinforce each other, creating a bright fringe. The dark bands, on the other hand, occur when the path difference is a half-integer multiple of the wavelength. In these regions, the waves arrive out of phase and cancel each other, resulting in a dark fringe.
The exact positions of the bright and dark fringes depend on the wavelength of light, the distance between the slits (known as the slit separation), and the distance from the slits to the screen (known as the screen distance). The fringe pattern becomes more pronounced as the slit separation decreases, the screen distance increases, or the wavelength of light decreases.
Young's double-slit experiment is a classic demonstration of the wave-particle duality of light, showing that light exhibits both wave-like and particle-like properties. The interference pattern observed in the experiment provides evidence for the wave nature of light and is a fundamental phenomenon in the field of optics.