In the Young's double-slit experiment, the interference pattern that is observed on a screen consists of a series of alternating bright and dark fringes. The central fringe, specifically, is typically the brightest fringe in the pattern. This brightness can be understood through the principle of superposition and constructive interference.
When light passes through the two closely spaced slits, it diffracts and creates two coherent wavefronts that act as sources of secondary wavelets. These wavelets propagate outward and overlap on the screen, leading to interference.
At the center of the interference pattern, the path lengths from both slits to the screen are approximately equal. Therefore, the waves from each slit arrive at the center in phase, meaning their crests and troughs align. When two waves arrive in phase, they undergo constructive interference, resulting in an intensity maximum or a bright fringe.
In this region, the amplitudes of the waves from both slits add up, reinforcing each other and creating a brighter region compared to the other fringes. The constructive interference at the center occurs because the path difference between the two slits is zero, resulting in a maximum amplitude of the combined wave.
As you move away from the center fringe, the path lengths start to differ, leading to varying path differences. This results in changing phases between the waves from the two slits, causing constructive or destructive interference at different points on the screen and creating the alternating bright and dark fringes in the interference pattern.
In summary, the center fringe is bright in the Young's double-slit experiment due to constructive interference. The waves from the two slits arrive at the center in phase, reinforcing each other and producing a maximum intensity.