In Young's double-slit experiment, when two slits are used, interference between the two coherent wavefronts gives rise to a pattern of bright and dark fringes on a screen. Each bright fringe corresponds to constructive interference, where the waves from the two slits arrive in phase, reinforcing each other. The dark fringes correspond to destructive interference, where the waves arrive out of phase, canceling each other.
Now, if three slits are used instead of two, the interference pattern becomes more complex. The central slit, referred to as the central maximum, acts as a reference point, producing a central bright fringe. The two adjacent slits on either side of the central maximum produce secondary maxima. These secondary maxima are not as bright as the central maximum but are brighter than the surrounding regions.
The reason for the alternate bright and dim fringes in the pattern with three slits is due to the interference between the waves from all possible combinations of the three slits. Each slit contributes its own set of secondary maxima, resulting in a more intricate interference pattern.
In the regions where the waves from two slits arrive in phase (constructive interference), bright fringes are observed. In contrast, when the waves from two slits arrive out of phase (destructive interference), dark fringes are observed.
The spacing between the bright and dark fringes depends on the relative distances between the slits and the screen, as well as the wavelength of the light used. The pattern will exhibit a regular alternation of bright and dark fringes due to the complex interference effects from the three slits.
Overall, the presence of three slits in Young's double-slit experiment leads to an interference pattern with alternate bright and dark fringes, reflecting the intricate superposition of wave amplitudes from multiple sources.