Read the passage below and answer the following questions in this context.

YDSE is abbreviation used for Young's Double Slit Experiment. YDSE shows interference of light. Interference of light is the phenomenon where two light waves meet each other while travelling in the same medium. A light source is kept behind a wall where two slits are made : S1 and S2. On the screen, there appears a pattern of alternate bright and dark fringes. The central fringe is always bright. 

Consider the interference between waves from two sources of intensities I and 4I. What is the intensity where the phase difference is  π?

I

$I =  { a_1}^{ 2}  + { a_2}^{ 2}  + 2 a_1 a_2 Cos \delta$

$a_1^2\propto  I$

$a_2^2\propto  4I$

δ = π