This Science Fair Project is about ‘after images and how they are produced. When you look at the sun in noon for few seconds and after that if you look into the surroundings, you can see dark images in front of your eyes which are called ‘after images’.
Materials Required for the project:
- A sheet of White paper
- A flashlight
- An Opaque tape
Procedure of the Project:
1. The white paper we have is taped over the lens of the flashlight and most of this paper is covered with strips of the opaque black tape. In order to shine the light through the paper, leave an area uncovered, which can be of any shape but should be at the centre of the lens.
2. Switch on the flash light in a dark room, hold it at arm’s length and shine it into your eyes.
3. Focus at any particular point of brightly lit shape for about 30 seconds.
4. Focus at a blank white wall and blink your eyes for few times. You can see an image which is called ‘after image’.
5. The shape and colour of the image seen on the wall should be noted.
6. Now close your left eye and focus to the bright image with the other eye for few seconds. Then close your right eye and look at the white wall with your left eye. Now you will not see an ‘after image’.
Scientific Explanation of the effect:
We see with the help of Retina which is the light sensitive lining at the back of our eye. A part of the retina is de-sensitized through prolonged stimulation by a bright image. Here it is the light source. The light reflecting from the wall shines when we look into the white wall. The part of the retina desensitized by the bright image will not respond to the new light input as its other sensitive parts do. This desensitized area appears as a negative ‘‘after image’ ’, a dark area that matches the original shape. The ‘after image’ may last for 30 seconds or longer.
The apparent size of the ‘after image’ depends on size of the image on your retina as well as on how far you perceive the image to be. If you use your hand instead of the white wall, you see your negative ‘after image’ on your hand. This is because, your hand is closer to you and so you see the image as relatively small, not larger than your hand. When you look at the distant wall instead of hand, you can see the negative ‘after image’ on it but the size is different from that of the hand. The size is bigger and the negative ‘after images’ covers a reasonable part of the white wall. This means that the negative ‘after image’ is not actually on either surface, it is on the retina. So the actual size of the ‘after image’ does not change but your interpretation of its size changes.