What is Young's double slit?

Answer 1

Two narrow slits with gap 0.03 mm and got separated by 3 mm will be illuminated by a monochromatic light which comes out of single slit. These two slits are named as Young's double slits. The waves coming out of these two will be in coherence.

Answer 2

Thomas Young was one of the first scientists to suggest that light behaves like a wave. In his double slit experiment, laser light is sent through two parallel slits and onto a screen a short distance beyond the slits. There is only one vertical line on the screen where the distance from each slit is the same. At that line the light from one slit is in phase with the the light from the other slit because the lengths of the two light paths are the same number of wavelengths. The light there is twice as intense as the light coming from one of the slits because when waves meet in phase their amplitudes are added together. Across the screen are places where the distances to both slits are multiples of a whole wavelength and places where the distance to one slit is a multiple of a whole wavelength but the distance to the other is half of a wavelength more or less than a multiple of a whole wavelength. At the 2nd set of places the light from one slit is 180 degrees out of phase from the light from the other, so they cancel each other out. If you give a wave a 180-degree phase shift it becomes the mirror image of the original. Where the waves are in phase it's like adding 1 & 1, and where they're completely out of phase it's like adding 1 & -1. So across the screen is what is called an interference pattern, alternating lines of brightness and darkness; proof that light travels in waves.