When two coherent sources interact with each other, there will be production of alternate bright and dark fringes on the screen . Young's double-slit experiment domonstrates the idea of making two coherent sources. For better visibility, one has to choose proper amplitude for the sources. The phenomena is good enough to satisfy the conservation of energy principle. The pattern formed in YDSE is of uniform thickness and is nicely placed on a long distance screen.
Law of convervation of energy is satisfied because

Options

A. equal loss and gain in intensity is observed
B. all bright fringes are equally brigth
C. all dark frings are of zero brigthness
D. the average intensity on screen is equal to the sum of intensities of the two sources

Answer

d

Explanation

`I_(av) = (I_(max) + I_(min))/(2)`
`= ((A_(1) + A_(2))^(2) + (A_(1) - A_(2))^(2))/(2)`
`= A_(1)^(2) + A_(2)^(2) = I_(1) + I_(2)`
`I_(av) = I_(1) + I_(2)`
So, choice (d) is correct.
Neither loss nor gain of energy is observed, but only redistribution of energy takes place.