Interference is the key mechanism behind the formation of a diffraction pattern in a grating.
A diffraction grating is made up of many closely spaced slits or grooves. When light passes through or reflects off these slits, each slit acts like a source of a new wave. These waves then overlap and interfere with each other.
At certain angles, the waves from different slits arrive in phase—their peaks and troughs line up. This creates constructive interference, resulting in bright spots on a screen. At other angles, the waves arrive out of phase, cancelling each other out. This causes destructive interference, leading to dark regions.
Because different wavelengths (colors) of light interfere constructively at different angles, the grating separates white light into its component colors, forming a spectrum. Each bright line in the pattern corresponds to a specific wavelength that satisfies the condition for constructive interference.
So, interference causes the light waves to reinforce or cancel out in specific directions, creating a structured pattern of bright and dark regions—this is the diffraction pattern seen in a grating.