Light rays
Have you ever woken early and not been able to get back to sleep again? If so, you may have noticed the following changes in your bedroom as you lay there. At first the room would have been very dark. Although you knew that there were many things in your bedroom you could not see any of them. Gradually the space around the top, bottom and sides of the curtains became lighter. When you looked around the room, you found that you could see the outline of large objects such as cupboards and wardrobes. As the space around the curtains became even brighter, some patches in the pattern of the curtains also let some light through into the room. Smaller objects such as a computer or clothes draped over a chair could now be seen. Most of the objects looked black or grey. A few, such as a coffee cup, appeared whiter than the rest. As more and more light entered the room you could see the colours of all the things in your room and probably decided to get up.

What had happened to change the light in the bedroom? It changed because of the turning Earth and the glowing Sun. All your life the Earth is turning beneath your feet. It turns round once in every twenty four hours. For part of this time, it turns the place where you live away from the Sun. When this begins to happen, the light fades from the sky. We call this time dusk. Later when the Earth has turned you further from the Sun all the sunlight fades away and you look up into a dark night sky. The Earth continues turning through the night and eventually, the place where you live is turned back towards the Sun. When this happens, the sky begins to brighten. We call this time dawn. Eventually, if the sky is cloudless, the Sun rises over the horizon and climbs higher in the sky throughout the morning.

Some people, when they were younger, thought the Sun went out at night and started glowing again in the morning. This is not so. The Sun has been glowing for about five billion years and may continue to do so for another five billion years. Night and day occur on Earth simply due to the turning of the Earth. When the place where you live turns towards the Sun it is daytime. When it turns away from the Sun, it is nighttime. The Sun keeps shining in space all the time.

You may wonder where sunlight comes from or indeed what the Sun really is. The Sun is a huge ball of gas a million times larger than the Earth. It is made from two gases called hydrogen and helium. They are held together by the force of gravity. In fact, the force of gravity is so strong that it squashes hydrogen at the centre of the Sun until it turns into helium. When this change takes place, heat and light energy are produced. The Sun is so huge that it takes a million years for this energy to travel from the centre of the Sun to the surface. When the energy reaches the surface, it spreads out in all directions through space. It takes just about eight minutes for the light and the heat from the Sun to reach the Earth.

Although we need light to see we very rarely see light on the move. It appears as an invisible something between us and everything around us. Occasionally you can see light on the move. If you have ever had your bedroom curtains slightly open you may have seen light streaming into the room. The reason you can see it is that some of it is being reflected off pieces of dust in the air. When you see light behaving like this, you see it travelling in a straight line. This line of light is called a light ray if it is thin and a light beam if it is thick.

So what really happened as you lay in your bed and watched the dawn arrive? The Earth turned you home towards the Sun. Light shot through space in straight lines from the Sun and entered your bedroom. The light rays struck the objects in your room and some of the rays bounced off the objects and into your eyes so that you could see. If you go to bed early, the reverse happens. The Earth turns your home away from the Sun and less and less light enters your bedroom. You probably would not see these changes though, because you would have gone to sleep.

Is the Sun the only light source in the sky?
No. All the stars are sources of light. They are made from hydrogen and helium like the Sun. Some stars are larger than the Sun and some are smaller. However they are all at such great distances from the Earth that they all look just like points of light.

Is the Moon a source of light?
No. It receives light from the Sun and some of this light is reflected towards the Earth. The different shapes of the Moon that we see, such as the crescent, are due to the way the Moon moves round the Earth. The Moon moves in an orbit round the Earth. At one point in the orbit the Moon faces the Sun and the Earth. When this happens we see a full round Moon called a full Moon. At other points in the Moon's orbit only part of the sunlit surface of the Moon faces the Earth and makes the shape of the Moon we see. These different shapes of the Moon's sunlit surface we can see are called the phases of the Moon.

What other sources of light are there?
When a substance burns, light energy is given out and so a lighted candle, a fire or a firework are sources of light. If a gas cooker is switched on the burning gas produces a flame which is a light source. In the past, gas was used to light homes. Today gas is used on campsites to provide light in some caravans and tents. Light is also produced when electricity flows through a wire made from tungsten in a light bulb. It is also made when a current of electricity flows through a strip light and makes the side of the light glow.
Television screens and computer screens glow owing to the activity of electricity and are also sources of light.

Why can white objects be seen in a room before other objects?
They reflect the most light in the room. The other objects reflect less light and cannot be seen until a greater amount of light enters the room.

Why cannot the colour of objects be seen straight away?
Our eyes have two kinds of structures which receive light. They are called rod cells and cone cells and both are found in the back of the eye called the retina. The rod cells begin sending messages to the brain when they detect any light but they cannot send any information about colour. The cone cells send information about colour but they cannot work until there is a large amount of light entering the eye. This means that the eye can detect the shapes of objects first as a small amount of light strikes the rod cells, then the eye detects colour later when a larger amount of light strikes the cone cells.

How does light really travel from one place to another?
It always travels in straight lines but scientists have found that the actual way it travels is a bit of a mystery. When certain experiments are tried it seems to behave as if it is made of waves. If other experiments are tried it behaves as if it is made of particles. These light particles are called photons. You may sometimes hear of the word photon in science fiction programmes but photons are real. Usually we talk about light as travelling as waves.

Are light waves like sound waves?
No. Sound waves are made by particles in the air moving to or fro. Light waves are made from a combination of electricity and magnetism. These waves are known as electromagnetic waves. The vibration of electricity and magnetism in the waves does not need the help of particles so the waves can travel through space as well as air. Sound waves, which need particles to help them move, cannot travel through space.