Light, like heat, is a form of energy and there is a close connection between the two. Most bodies (i.e. objects) which emit, or send out, light also emit heat. The Sun, for example, emits both light and heat energy. So does an electric lamp or a fire. We can see these bodies because they give out light. They are called luminous bodies. The Moon, however, is not a luminous body because it does not produce light. We can see the Moon in the sky only because it reflects the light from the Sun. This means the Sun sends out light to the Moon and the Moon sends the Sun's light back to Earth. So, the Moon is a good example of a non-luminous body. Because it is non-luminous, it does not produce heat. When there is no sunlight, it is very cold there. The temperature can be 129°C below zero. A piece of iron or glass is also non-luminous. W fiat are transparent, translucent and opaque substances? So, light is an important source of energy. We can see things only because there is light from luminous bodies. But light cannot pass through every substance. For example, we can see through clear glass or water but we cannot see anything behind a wall. Substances such as clear glass, water or some kinds of plastic are called transparent substances. That is, light can pass through them, and we can see what is on the other side. These transparent substances can be used for different purposes. For example, windows are made of glass because we want to be able to see what is outside. But some substances are not clear; in other words, they are not transparent. We cannot see the objects behind them but we can only see the light. These are called translucent substances. Some types of glass are like this. There are also substances called opaque. Light cannot pass through such substances and we cannot see anything behind them.