Question

A student is asked to measure the wavelength of monochromatic light . He sets up the apparatus as shown. $S_1,S_2,S_3$ are narrow parallel slits. $L$ is radiant lamps and $M$ is a micrometer eyepiece. The student fails to observe interference fringes. We would advice him to

• A. increase the width of $S_1$
• B. decrease the distance between $S_2$ and $S_3$.
• C. replace $L$ with a white light lamp.
• D. make $S_2$ and $S_3$ wider.

Answer 1

The correct option is B decrease the distance between $S_2$ and $S_3$.

The fringe width of a double slit interference pattern is given by

$\beta =\frac{D\lambda }{d}$

where $D$ is the distance between screen and the plane containing slits

$d$ is distance between the slits $S_2$ and $S_3$.

To make the fringe pattern visible, the fringe width must be increased, for which $d$ should be decreased.