Concepts of Physics

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

Diffraction from a Thin Wire

Objective

To view the diffraction pattern when light is obstructed by a narrow wire.

Introduction

When a monochromatic light passes around a fine wire, a diffraction pattern is observed at the screen placed on the other side of the wire. The intensity at the central point is large and decreases on the sides till it becomes zero. But then it again increases as you go further away from the centre, becomes maximum and then decreases again. It goes on and you get a series of bright and dark strips called fringes. The central bright fringe is very intense and successive bright fringes are lesser and lesser intense. In fact, the diffraction pattern is similar to what is observed when a single slit is used. You can also use human hair in place of the wire.

Apparatus

A laser torch, metallic wire.

Procedure

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Tighten the metallic wire in vertical position. The wire should be at a distance of about 2 metre from the wall which is to be used as screen. Put on laser torch so that light falls on the wire and its direction is perpendicular to the wall. See the diffraction pattern on the wall. You will see bright and dark fringes.

Discussion

If you have done the experiment on diffraction from a single slit, you will immediately notice that the fringes in this case are very wide as compared to that formed with the single slit.

There is a principle which says that the diffraction due to an obstacle is identical to that due to an opening of the same size. Thus the diffraction pattern due to a single wire is identical to that due to a single slit of the same size. As the thickness of the wire is very small, the equivalent slit width is also small and hence diffraction fringes are wide.