In Young's double-slit experiment, the two slits act as coherent sources of equal amplitude `A` and of wavelength `lamda`. In another experiment with the same set-up the two slits are sources of equal amplitude `A ` and wavelength `lamda`, but are incoherent. The ratio of the intensity of light at the midpoint of the screen in the first case to that in the second case is....

Answer

B

Explanation

For coherent sources, for constructive interface The amplitude at the mid point `= A+A=2A`
`implies I_1propto (2A)^2 implies I_2propto 4I_0` ….(i)
`NOTE: For incoherent sources, the intensity add up normally (no interface).
There fore, the total intensity `i_2=2I_0` ....(ii)
From (i) and (ii) `(I_1)/(i_2)=(4I_0)/(2I_0)=2`