IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# To make double-slit interference pattern

## Introduction

Young's double slit interference experiment done in the year 1801-03 is one of the most beautiful and impactful experiments in the history of Physics. It is this experiment which established wave theory of light in most convincing way and resolved about 130 years of debate between wave and particle theory. However, it is somewhat tricky to make a narrow double slit with appropriate separation between these to properly observe interference pattern. The text book description of YDSE assumes the individual slits to be of very small size so that the diffraction patterns of the slits is totally neglected. Practically, the double slit pattern will be convolution of interference pattern by the diffraction pattern envelop as illustrated in the figure. The separation between successive interference fringes is given by $$\sin\theta=\lambda/d$$ and that between successive diffraction fringes is $$\sin\theta=\lambda/b$$ . Here $$b$$ is width of the individual slits and $$d$$ is the separation between the centers of the slit. The central fringe in different pattern is about double as wide as other fringes.

## Apparatus

A CD, two ring magnets, a razor blade, a piece of nichrome wire, a stand to keep the CD is a vertical plane, a laser with a stand, a distant screen.

## Procedure

1. Make two narrow holes in a CD along a diameter, each hole at a distance of about 2 cm from the center. Push small nut bolt in each hole.
2. Making the double slit:
• Fix the nichrome wire in stretched position across the central hole in the CD in the nut bolts.
• Put two ring magnets one on each side of the central hole on one surface of the CD, one in each nut-bolt. As the nut bolt is ferromagnetic, the magnets will get fixed.
• Divide the razor blade in two parts braking it along the longer side.
• Put the two parts on the two sides of the hole on the other surface of the CD (magnets and the blade are on opposite surfaces). Because of the magnets, the blades stick to the CD.
• Now carefully slide the 4 ends of the blade pieces to make the two edges parallel to the wire and as close as possible. Your double slit is ready.
3. Put the CD on its stand so that the double-slit is vertical.
4. Shine the laser on the double slit to illuminate the wire and the two slits.
5. Look at the pattern on the screen at a large distance.
• Question: Do you see central bright fringe much wider than the adjacent bright fringes or do they have the same width?
• Question: Do you see the central bright fringe more intense than the adjacent bright fringes.
• Question: Identify diffraction maxima. Between two consecutive diffraction maxima, how many extra interference maxima are seen?
6. Remove the blades from the vicinity of wire. Put the laser on the wire and see the pattern.
7. How does this single wire pattern differ from the double slit pattern?