# Diffraction

The bending of light through small openings (or obstacles) is called diffraction. It is visible when size of the opening is comparable to the wavelength $\lambda$ of light.

## Diffraction from a thin wire

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.

### Procedure

You need a laser torch and a metallic wire.

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.

## Diffraction from Single Slit

When a monochromatic light passes through a narrow slit, a diffraction pattern is formed on the screen placed on the other side of the slit.

### Procedure

You need a razor blade and a laser torch for this experiment.

Make a single slit of width few hundred microns by mounting two half pieces of razor blades side by side. Fix this single slit in a vertical position. Put on laser torch and adjust so that light passes through the empty space. The direction of the laser should be perpendicular to the slit. Observe the light falling on the wall.

### Discussion

You will see the diffraction fringes, very bright centre and then bright spots with gradually decreasing intensity, separated by dark spots. The number of fringes is large due to intense beam of laser torch.

Formation of alternate bright and dark fringes when a monochromatic light passes through a single slit is a strong evidence of wave nature of light.

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