Question

In YDSE experiment a glass plate of refractive index 1.5 is placed infront of one of the slits. Minimum thickness 't' of this glass plate so that intensity of light at position of central maxima [before glass plate] remains unchanged is

• A. $\frac{\lambda }{2}$
• B. $\frac{\lambda }{3}$
• C. $\frac{2}{3}\lambda$
• D. $2\lambda$

Answer 1

The correct option is D $2\lambda$

Path difference due to plate is

$\Delta x=(\mu -1)t$

For constructive interference $\Delta x=n\lambda$

$n\lambda =(\mu -1)t$

For $t_{min}=\frac{\lambda }{(\mu -1)}=2\lambda$