Chapter 22, Problem 1RQ

To determine

The reason why light is projected upside down on the wall behind the screen, when the light source is placed in the front of a screen with a small hole.

Answer to Problem 1RQ

Solution:

Light always travels in straight line, so it has to cover the same distance in same time. Due to this, the rays from the bottom portion of the light source pass through the hole and reach the wall, and other rays from the top portion of the light reach the wall but below the wall and it forms an inverted image on the wall.

Explanation of Solution

Introduction:

Experimentally, it has been observed that, the projection on the wall will be upside down when a light source placed in front of screen with a small hole, because the light always travels in straight line through the small hole.

Explanation:

From the schematic diagram shown below, consider the ray 1 passes from the bottom of the flame and the ray 2 passes from the top of the flame. The ray 1 passes from the bottom of the flame, reaches the wall on some distances from the wall in a dark room, and a stiff paper with small hole is placed in between the candle and wall.

If source of light sends the light in all direction; from the source of light, multiple rays will emit and reaches to stiff paper. In addition, due to stiff paper, some light rays were blocked and some lights were reached on screen from the top and the bottom portion of light source.

In such a way, the rays from the bottom portion of the light pass through the hole and reach the wall, and other rays from the top portion of the light reach below the wall where the light rays can be seen in the above diagram. Because the light always travels in straight line, so it has to cover same distance in same time. And due to this, the projection will be seen on the wall upside down.

Conclusion:

The projection will be upside down because the light always travels in straight line through the small hole. And due to this, the rays from the bottom portion of the light pass through the hole and reach to the wall, and other rays from the top portion of the light reach the wall but below where the light rays from bottom portion did, it can be seen in the above diagram.