Interference fringes are produced on a screen by using two light sources of intensities / and 9/. The phase difference between the beams `pi/2` is at point P and `pi`at point Q on the screen. The difference between the resultant intensities at point P and Q is

Options

A. 2
B. 4
C. 6
D. 8

Answer

C

Explanation

According to question, resultant intensity of interferring wave is given by
`l_P=l_2+2sqrt(l_1l_2cosphi)`
For `phi=pi/2,l_P=l+9l=10l`
Again at point Q, resultant intensity is given by
`l_Q=l_1+l_2+2sqrt(l_1l_2cosphi)`
`phi =pi,l_Q=l+9l+(-2sqrt(9(l)^2)`
`=10l-6l-4l`
Now, difference between the resultant intensity is given by `Deltal=l_P-l_Q=10l-4l=6l`