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

Light through a small hole of specific geometry

Introduction

If you look at the sunlight coming through the leaves of a tree and falling on the ground, you find similar looking circular or elliptical spots. The geometry of the spaces between the leaves does not show up in the pattern. This experiment creates a similar situation in the classroom and stimulates discussion.

Apparatus

A bulb holder with aperture limiting slit attached, a filament bulb, a movable white screen, cardboards with holes of different shapes, such as triangle, rhombus, plus sign etc.

Procedure

1. Place the bulb in the holder and place it on a table. Place the screen in front of the bulb at a distance about of 20 cm. The length of the filament should be perpendicular to the line joining the bulb and the screen.
2. Now bring a cardboard with hole between the bulb and the screen so that light only passes through the hole. Keep the cardboard close to the screen, say at 4-5 cm. See the shape of the light spot on the screen. Is it same as the shape of hole? Try with different cardboards and see that each time the spot has the same shape as the hole.
3. Keeping the cardboard at the same place, displace the screen away from the cardboard. Take it up to 60 cm. How does the shape of the spot change?
4. Use different cardboards and compare the shapes when its distance from the screen is large. Write your conclusions.