In the Young's double slit experiment, when both slits are open, a pattern of interference is observed on the screen. This interference pattern arises from the superposition of waves from the two slits, leading to constructive and destructive interference.
However, when one of the slits is closed, the interference pattern disappears, and a different pattern known as a diffraction pattern is observed instead. This change occurs because the interference requires the superposition of waves from both slits, which allows for the constructive and destructive interference. When one of the slits is closed, there is no longer a second source of waves to interfere with, and thus no interference occurs.
Closing one of the slits effectively changes the experiment from a double slit experiment to a single slit experiment. In a single slit experiment, the light passing through the single open slit diffracts, spreading out and creating a central maximum surrounded by narrower secondary maxima and minima on the screen. This pattern is characteristic of diffraction, where the light waves bend around the edges of the slit.
In summary, when one of the slits is closed in the Young's double slit experiment, interference does not occur because the superposition of waves from both slits is necessary for interference. Instead, a diffraction pattern is observed due to the bending of light waves around the edges of the single open slit.