Why are interference fringes equally spaced in certain setups?

Interference fringes are equally spaced in certain setups due to the constant path difference between two interfering waves.

In physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude. This occurs when waves from two coherent sources meet and their phases either reinforce or cancel each other. The points of reinforcement result in bright fringes, while the points of cancellation result in dark fringes. These fringes are observed in experiments such as Young's double-slit experiment and are a direct result of the wave nature of light.

The spacing of these fringes is determined by the path difference between the two interfering waves. The path difference is the difference in the distances two waves must travel from their sources to a given point. If this path difference remains constant, as it does in certain experimental setups, the interference pattern will consist of equally spaced fringes. This is because the phase difference between the waves, which determines whether they will interfere constructively (forming a bright fringe) or destructively (forming a dark fringe), is directly related to the path difference.

In Young's double-slit experiment, for example, the path difference is determined by the spacing of the slits and the angle at which the light hits the screen. If these factors remain constant, the path difference will also remain constant, resulting in equally spaced fringes. The same principle applies to other interference experiments, such as those involving thin films or diffraction gratings.

In conclusion, the equal spacing of interference fringes in certain setups is a result of the constant path difference between the interfering waves. This path difference determines the phase difference between the waves, which in turn determines whether they will interfere constructively or destructively. As long as the path difference remains constant, the interference pattern will consist of equally spaced fringes.