In Young's Double Slit Experiment, when both slits are open, light passes through both slits and creates an interference pattern on a screen placed behind the slits. This pattern is characterized by alternating bright and dark fringes resulting from the constructive and destructive interference of the light waves.
When one of the slits is covered, the interference pattern disappears because the conditions necessary for interference are no longer met. Interference occurs when two or more coherent light waves overlap and interfere constructively or destructively.
In the double slit experiment, the light waves passing through both slits interfere with each other. This interference is based on the relative phase of the waves. When both slits are open, the waves from each slit travel different paths and then overlap on the screen. If the paths from the slits to the screen have a consistent phase relationship, the waves can constructively interfere, creating bright fringes. However, if the paths have an inconsistent phase relationship, the waves can destructively interfere, resulting in dark fringes.
When one of the slits is covered, only one path of light is available, and there is no longer a second path for interference. The light passing through the open slit will create a diffraction pattern, but there will be no interference pattern because there is no second wave to interact with. Consequently, only a single-slit diffraction pattern will be observed.
In summary, the interference pattern disappears when one of the slits is covered because interference requires the interaction of two coherent light waves, and with one slit covered, only a single wave is present, resulting in a diffraction pattern instead.