Q 12-10-010JEE MainAIEEE 2011Medium
In Young's double slit experiment, the two slits act as coherent sources of waves of equal amplitude $A$ and wavelength $\lambda$. In another experiment with the same arrangement the two slits are made to act as incoherent sources of waves of same amplitude and wavelength. If the intensity at the middle point of the screen in the first case is $I_1$ and in the second case $I_2$, then the ratio $\dfrac{I_1}{I_2}$ is
Answer: (B) $2$
Concept. Coherent waves add amplitudes; incoherent waves add intensities. At the centre the path difference is zero.
Step 1. Coherent. Amplitude $2A$, so $I_1\propto4A^2$.
Step 2. Incoherent. $I_2\propto A^2+A^2=2A^2$.
Step 3. Ratio. $\dfrac{I_1}{I_2}=2$.
Answer. $2$ (Option B).
Solution by Sreeraj P, M.Sc Physics
