In order to observe the interference of light, the dimensions of the slits must be on the same order of magnitude as the wavelength of light. This is known as the principle of wave-particle duality, which states that light has both wave-like and particle-like properties. When light passes through a pair of narrow slits, it diffracts and creates an interference pattern on a screen behind the slits. This interference pattern is a result of the constructive and destructive interference of the light waves. The spacing between the slits determines the distance between the bright and dark fringes of the interference pattern.