In Young's, double slit experiment, green light is incident on two slits. The interference pattern is…
In Young's, double slit experiment, green light is incident on two slits. The interference pattern is observed on a screen. Which one of the following changes would cause the observed fringes to be more closely spaced?
Reducing the separation between the slits
Using blue light instead of green light
Using red light instead of green light
Moving the screen away from the slits
Solution
The formula for fringe width is $W=\frac{\lambda D}{d}$
$\therefore \quad \mathrm{W} \propto \lambda, \mathrm{W} \propto \mathrm{D}$ and $\mathrm{W} \propto \frac{1}{\lambda}$
If the distance from the screen is increased, the width will increase.
If the distance between the slit is decreased, the width will increase.
As the wavelength will decrease the distance between the fringes will decrease.
$\lambda_{\text {nd }}>\lambda_{\text {erex }}>\lambda_{\text {max }}$
$\therefore \quad$ Blue light should be used.