Light always travels in a straight line.
When light cannot pass through an object, a dark patch called shadow is formed at the back of an object.
Shadow is always formed at the opposite side of a light source. A light source and an opaque object is needed to form a shadow.
When you enter a dark room and light a torch on an object, you can notice a shadow forms at the back of an object on the wall. Shadows are visible only on the screen-like objects such as the wall of a room, building, ground, etc.
Whatever may be the colour of an object, the shadow will always be dark in color, and it may not give correct information about the shape of an object.
Pinhole camera works on the principle that light travels along a straight line. Here, you can see that a small inverted image is formed when compared to the original object. Even an eclipse can be viewed through a pinhole camera.
Steps to make a sliding pinhole camera
1. Take two cardboard boxes, one smaller in size than the other so, that we can place one box into another without any gap in between them.
2. Make a small hole in the middle of the larger box and cut open the other side.
3. Now, cut a square-shaped piece at a size of about \(5\) to \(6\ cm\) from the middle of the smaller box and cover it with translucent screen or paper.
4. Slide the smaller box into the larger one keeping the translucent screen or paper inside. Now, a pinhole camera is ready.
5. Take a black cloth to cover your head and the pinhole camera.
6. Look through the open side of the smaller box and focus at a distant object in the bright sunlight.
7. Adjust the smaller box till you get the image at the translucent screen or paper.
Pinhole camera in nature
A small patches of sunlight is visible under a tree with large leaves and thick branches. The circular images formed between the gaps of the leaves, act as pinhole image of the Sun.
When you place a lighted candle at one end of the table, you can view it through a rubber pipe standing at the other end. But, when you bend the rubber pipe, you cannot view the lighted candle, since light travels in a straight line.
Reflection in a mirror
The bouncing of light from a smooth surface is called reflection. Smooth polished surfaces called mirrors can reflect light falling on it in any direction. The reflection from a mirror gives a clear image of an object.
In a dark room, place a mirror at one corner and pass a direct beam of light from a lighted torch standing from another corner. You can now notice a patch of light falling on to the other side of the room. When you adjust the direction of the lighted torch, the patch of light from the mirror also changes its direction showing that light travels along a straight line.