In Young's double-slit experiment, the interference pattern is indeed obtained by passing light through two slits and observing the resulting pattern on a screen. If you use only one slit instead of two, you will not observe the characteristic interference pattern. Instead, you will see a diffraction pattern.
Diffraction occurs when light waves encounter an obstacle or aperture that causes them to bend or spread out. When light passes through a single slit, it diffracts, and the resulting pattern on a screen is a central bright region called the central maximum with alternating dark and light regions on either side. This pattern is known as a single-slit diffraction pattern.
The reason for this diffraction pattern is that light waves passing through a single slit interfere with each other, leading to constructive and destructive interference. The central maximum is brighter because the light waves passing through the center of the slit experience minimal diffraction and exhibit constructive interference.
While a single slit produces a diffraction pattern, the characteristic interference pattern observed in Young's double-slit experiment arises from the interaction of light waves passing through two closely spaced slits. The light waves interfere with each other constructively or destructively, creating a pattern of bright and dark fringes on the screen.
To obtain the interference pattern in Young's double-slit experiment, it is essential to have two slits to generate the necessary interference effects.