Question

The figure shows two points source which emit light of wavelength $\lambda$ in phase with each other and are at a distance d = 5.5$\lambda$ apart along a line which is perpendicular to a large screen at a distance L from the centre of the source. Assume that d is much less than L. Which of the following statement is (are) correct?

• A. Only five bright fringes appear on the screen above (0, 0)
• B. Only six bright fringes appear on the screen above (0, 0)
• C. Point y = 0 corresponds to bright fringe
• D. Point y = 0 corresponds to dark fringe.

Answer 1

Answer: (A), (D)

The correct options are
A Only five bright fringes appear on the screen above (0, 0)
D Point y = 0 corresponds to dark fringe.

Since $\Delta x=\left(5.5\right)\lambda =\left(\frac{2\left(6\right)-1}{2}\right)\lambda$ we will have a minima at the centre of the screen.
All other path differences will be less than 5.5$\lambda$.
Thus, on one side of the central minima we will have five maxima.