Diffraction occurs at a double slit due to the interference of light waves.

When light waves pass through a double slit, they undergo diffraction and subsequently interfere with one another. This interference produces a distinctive pattern of bright and dark fringes on a screen positioned behind the slits. The bright fringes appear when the waves from each slit are in phase, meaning that their peaks and troughs align perfectly, resulting in constructive interference. Conversely, the dark fringes arise when the waves are out of phase, leading to destructive interference.

The spacing between these fringes is determined by the distance between the slits and the wavelength of the light. This relationship is described by the diffraction grating equation, which is expressed as

where:

• $d$ is the distance between the slits,
• $\theta$ is the angle between the screen and the incident light,
• $m$ is the order of the fringe, and
• $\lambda$ is the wavelength of the light.

The double slit experiment is a classic demonstration of the wave nature of light, illustrating the principles of diffraction and interference. It has also been employed to explore the properties of other types of waves, such as electrons and atoms. This experiment has significantly contributed to the development of quantum mechanics and has deepened our understanding of the fundamental nature of matter and energy.