Question

When light from two sources (say slits `S_(1)` and `S_(2)`) interfere, they form alternate dark and bright fringes. Bright fringe is formed at all point where the path difference is an odd multiple of half wavelength. At the condition of equal amplitudes, `A_(1) = A_(2) = a`, the maximum intensity will be `4 a^(2)` and the visibility improves, The resultant intensity can also be indicated with phase factor as `I = 2 a^(2) cos^(2) (phi // 2)`.Using this passage, answer the following questions.
If the slits `S_(1)` and `S_(2)` are arranged as shown in Fig. the ratio of internsity of fringe at P and R is

Answer 1

Correct Answer - a
Since the ligth from slit `S_(1)` and `S_(2)` reach the point P with a path difference of `lambda // 2`, the point P will be dark, `I = 0`. Since path difference is zero for R, there will be a bright fringe at R. So, ratio of intensity will be zero. So choice (a) is correct.