How do waves interfere in a Young's double-slit setup?

In a Young's double-slit setup, waves interfere constructively and destructively due to differences in their path lengths.

In a Young's double-slit experiment, light is shone through two closely spaced slits, creating two coherent light sources. These light sources produce waves that spread out and overlap on the other side of the slits. The interference pattern observed is a result of the superposition of these two waves.

When the waves meet, they interfere with each other. If the crest of one wave meets the crest of the other, they interfere constructively, resulting in a bright fringe. This is because the amplitudes of the two waves add together, producing a wave with greater amplitude. On the other hand, if the crest of one wave meets the trough of the other, they interfere destructively, resulting in a dark fringe. In this case, the amplitudes of the two waves cancel each other out, producing a wave with smaller or zero amplitude.

The pattern of bright and dark fringes is determined by the difference in path lengths from the two slits to the point of observation. If this path difference is a whole number of wavelengths, the waves will arrive in phase and interfere constructively, producing a bright fringe. If the path difference is a half-integer number of wavelengths, the waves will arrive out of phase and interfere destructively, producing a dark fringe.

The Young's double-slit experiment is a classic demonstration of the wave nature of light and the principle of superposition. It provides clear evidence of interference, a phenomenon that can only be explained if light is considered as a wave. The experiment also allows us to measure the wavelength of light, by analysing the interference pattern and knowing the separation of the slits and the distance to the screen.