A student is performing Young’s double slit experiment. There are two slits S1 and S2. The distance between them is d. There is a large screen at a distance D (D >> d) from the slits. The setup is shown in the following figure. A parallel beam of light is incident on it. A monochromatic light of wavelength λ is used. The initial phase difference between the two slits behave as two coherent sources of light is zero. The intensities of light waves on the screen coming out of S1 and S2 are same, i.e. Io. In this situation, the principal maximum is formed at point P. At the point on screen where the principal maximum is formed, the phase difference between two interfering waves is zero.
For paraxial rays, focal length approximately is
(A) R
(B) R/2
(C) 2R
(D) none
