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Why does closing one slit in Young’s experiment remove the interference fringes?

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A horizontal gradient bar displays the visible light spectrum from violet to red, labeled with corresponding frequency in terahertz, wavelength in nanometers, and photon energy in electron volts. — light When white light is spread apart by a prism or a diffraction grating, the colours of the visible spectrum appear. The colours vary according to their wavelengths. Violet has the highest frequencies and shortest wavelengths, and red has the lowest frequencies and the longest wavelengths.

In Young’s double-slit experiment, the interference fringes are created by the superposition of light waves emerging from two separate slits. These waves must overlap and maintain a consistent phase relationship to produce the alternating pattern of constructive and destructive interference.12

When you close one of the slits, you remove one of the two sources of light waves. Because there is no longer a second wave to overlap with the first, the superposition principle cannot produce an interference pattern. Instead of the fringe pattern, you observe only the diffraction pattern associated with the single remaining slit. Essentially, both slits are required to create the interference fringes; without the second source, the mechanism for reinforcement and cancellation of light waves is eliminated.32