The dark bands observed in Young's double-slit experiment are a result of interference. In this experiment, a beam of light passes through two closely spaced slits, creating two coherent sources of waves. These waves then overlap and interfere with each other, resulting in a pattern of alternating bright and dark bands on a screen placed behind the slits.
The interference arises due to the superposition principle of waves. When the waves from the two slits meet, they can either reinforce each other (constructive interference) or cancel each other out (destructive interference), depending on their phase relationship.
At certain points on the screen, the waves from the two slits arrive in phase, meaning their crests and troughs align and add up constructively, resulting in bright fringes. These are the locations where the wave amplitudes add up, producing a maximum intensity of light.
On the other hand, at other points, the waves arrive out of phase, causing their amplitudes to cancel each other out and resulting in dark fringes. These are the locations where the wave amplitudes subtract from each other, producing a minimum intensity of light.
The specific positions of the bright and dark fringes can be calculated using the principles of wave interference. The spacing between the fringes depends on factors such as the wavelength of the light used, the distance between the slits, and the distance between the slits and the screen.
Overall, the dark bands observed in Young's double-slit experiment are a consequence of the destructive interference between the waves from the two slits, resulting in regions of reduced or no light intensity. This interference pattern provides direct evidence of the wave nature of light and is a fundamental aspect of wave phenomena.