Q. In Young’s double slit experiment, the two slit act as coherent sources if equal amplitude A and wavelength$\lambda$. In another experiment with the same setup, the two slits are sources of equal amplitude $A$ and wavelength $\lambda$ but are incoherent. The ratio of the intensity of light at the mid-point of the screen in the first case to that in the second case is
A) 2:1
B) 1:2
C) 3:4
D) 4:3
Explanation
(a) When sources are coherent
$I_{R} = I_{1} + I_{2} + 2\sqrt{I_{1}I_{2}}\cos\phi$
At middle point of the screen, $\phi = 0{^\circ}$
$I_{R} = I + I + 2\sqrt{II}\cos{0{^\circ} = 4I}$
When sources are incoherent,
$I_{R}^{'} = I_{1} + I_{2} = I + I + 2I$
$\frac{I_{R}}{I_{R}^{'}} = \frac{4I}{2I} = 2$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.