Young’s Experiment
In 1801, Thomas Young was the first who demonstrated the interference of light. In his experimental arrangement (as shown in the figure), monochromatic light (single wavelength)from a narrow vertical slit S falls on two other narrow slits ${\mathrm{S}}_1$ and ${\mathrm{S}}_2$which are very close together and equidistant to S.${\mathrm{S}}_1$ and ${\mathrm{S}}_2$ act as two coherent sources (both being derived from S). The emerging beams spread into the region beyond the slits .Superposition occurs in the shaded area, where the beams overlap. Alternate bright and dark equally spaced vertical bands (interference fringes) can beobserved on a screen placed at same distance fromthe slits. If either of S1 or S2 iscovered, then the fringes disappear.

(i) Suppose while performing double slit experiment, the space between the slits and the screen is filled with water. How does the interference pattern change? 

(ii) In Young’s double slit experiment, if the distance between the slits is halved, what changes in the fringe width will take place? 

(iii) The ratio of the widths of two slits in Young’s double slit experiment is 4:1.
Evaluate the ratio of intensities at maxima and minima in the interference pattern.