Hand Through A Glowing Bulb - A Science Fair Project

By Author: Jason john
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This experiment is a trick using a concave mirror. May be you feel it like a magic. You can see a glowing bulb in this experiment, but when you try to touch it, your hand will just pass through the bulb. You can just try it out.

What we need?

1. A 16 inch (40cm) diameter concave mirror
2. Wood to build a support stand for the mirror
3. Cardboard to make a small box for the light bulb
4. 2 light sockets (One with an electrical cord and plug)
5. A light bulb, 40 to 75 watts
6. One 2x5 inch (8x13 cm) card

What to do?

1. Using the cardboard, make a small box with one side open. The height of the box should be slightly less than that of the mirror. Also the box should be wide enough to accommodate the bulb. All the surfaces of the box should be painted with black.
2. There will be two sockets to be mounted, one with electrical cord and the other without cord. The socket without any electrical cord should be mounted in such a way that it is centred on the top of the box, right side up.
3. The other socket with the cord and plug should be mounted upside-down ...
... inside the box, directly beneath the empty socket, as shown in the figure 2.
4. As shown in the figure, build the mirror support and place the mirror in it.
5. Place the box with its open side facing the mirror after plugging the light bulb.
6. Place the mirror at least 20 feet (6m) far from the light bulb in such a way that its concave (hollow) side facing the light bulb. This is to find the focal length of the mirror. Close to the focal point of the mirror, the mirror will make a real image of the light bulb. The distance between the centre of the mirror and the image of the light bulb should be found out and then you will find the focal length of the mirror.
7. For finding the location of the image, take a 3x5 inch (8x13 cm) card and hold it near the centre of - and touching - the surface of the mirror. Move the card slowly away from the centre. The card will near to the focal point when the image of the bulb is in sharp focus.
8. Place the mirror two focal lengths from the light bulb. The image of the glowing bulb will be reflected by the concave mirror. The image will be formed in the space in front of the mirror. You can position the image so that it appears to be in the empty socket on the top of the box. This can be done by adjusting the vertical and horizontal position of the box carefully. (You can place magazines or books under the box to adjust its height.)
9. You can do this experiment in a dark room since this illusion works better in dark room. Make the people to stand back about 5 m from it so that they see the bulb in the upper socket. After that ask the people to move slowly towards the bulb. To maintain an undistorted image, they may have to bend or straighten or move sideways. Pass your hand through it, when they are about 6 feet (2 m) from the image of the bulb. Even everything is out in the open; the illusion of your hand passing right through a light bulb is quite impressive.

How does this happen?

The image you see is formed by the concave mirror. The light rays spreading out from one point on the light bulb are reflected by the concave mirror. Then they come back together at a point in space which creates a real image of that point. These rays after reaching our eyes fall on our retina and thus we are able to see the image. The difference between a retinal image of the reflected bulb and a retinal image of the actual bulb cannot be identified by our eye and brain.