In Young's double-slit experiment, the screen gets illuminated by light due to the phenomenon of interference. The experiment involves a beam of light passing through two closely spaced slits, creating two coherent sources of light waves. These waves then propagate outward and interact with each other, leading to the formation of an interference pattern on the screen.
When the waves from the two slits reach the screen, they superpose or combine with each other. At certain points on the screen, the crests (peaks) of one wave align with the crests of the other wave, leading to constructive interference. This results in the waves reinforcing each other, producing bright regions on the screen where the intensity of light is high.
Conversely, at other points on the screen, the crests of one wave align with the troughs (dips) of the other wave, leading to destructive interference. In these regions, the waves cancel each other out, resulting in dark regions or areas of reduced intensity on the screen.
The interference pattern that forms on the screen is a result of the varying path lengths between the two slits and different points on the screen. The constructive and destructive interference between the waves create alternating bright and dark fringes, known as interference fringes or bands.
Therefore, the illumination of the screen in Young's double-slit experiment is a consequence of the constructive interference of light waves from the two slits at certain points, leading to the accumulation of light intensity, while destructive interference results in reduced or no illumination at other points.