In a double-slit experiment, the phenomenon of dark and light bands is created through the interference of light waves. When light passes through two closely spaced slits, it behaves as a wave and undergoes diffraction. Diffraction is the bending or spreading out of waves as they encounter an obstacle or pass through an aperture.
When a beam of light passes through the two slits, it diffracts and creates two separate wavefronts that spread out and overlap behind the slits. These two wavefronts then act as coherent sources of light, meaning they have a constant phase relationship and can interfere with each other.
Interference occurs when the crests and troughs of the waves align or cancel each other out. Where constructive interference happens, the crests of one wave overlap with the crests of the other wave, resulting in bright regions called interference maxima. Conversely, destructive interference occurs when the crests of one wave overlap with the troughs of the other wave, leading to regions of darkness known as interference minima.
The specific pattern of dark and light bands on a screen placed behind the double slits is due to the varying path lengths traveled by the waves from the two slits to different points on the screen. These path length differences determine whether the waves are in phase (constructive interference) or out of phase (destructive interference) when they reach the screen.
The resulting pattern of bright and dark bands is known as an interference pattern and is a characteristic feature of wave-like behavior. It demonstrates the wave-particle duality of light, where light exhibits both particle-like (photons) and wave-like properties.