Beams of light
Has this happened to you? A favourite film is to be on television. You have look forward to it all day. In the evening you settle yourself in a comfy chair ready for the film to begin. Just as the title of the film comes on the screen, the screen goes black. In fact the lights go out, an electric fire goes out, the clock on the video recorder goes out. Everything is in darkness. In other parts of the house you can hear people complaining as they try light switches that don't work. There has been a power cut.

If you were in a power cut what would you do? You would not sit in the darkness because you might be there for hours. Eventually you may want something to eat or drink or maybe decide to go to bed. Moving around the home in the dark can be difficult. You bump into furniture and struggle to find door handles. Eventually you remember that you have a torch in a drawer nearby. You grope your way to the drawer and pull it open. You rummage among the contents of the drawer until your hands feel the torch. You pull it out, turn it to point into the room and switch it on.

Immediately a bright circle of light appears on the opposite wall. Furniture in front of the wall can be seen. You move the torch to the left and the circle of light travels along the wall to the left. You move the torch up and the circle of light settles on the ceiling. When you point the torch down the carpet is lit up. Now you can move around your home more easily. Wherever you want to go, you just point the torch and the way ahead lights up.

What happens when you switch on a torch? Inside a torch are batteries. They are stores of electricity. When the torch is switched on, electricity flows out of the batteries and around the wires in the torch. One of the wires is in the centre of the torch bulb. This wire is made of a special metal. When electricity flows through this wire, it gets hot very quickly and shines with a white light. The bulb is at the centre of a shiny dish. The surface of the dish acts like a mirror. It reflects the light away from the torch. You cannot normally see the light travelling away from the torch. The next time you see the light is when it makes a bright circle on any surface it strikes.

A car headlight is like a torch. It has a bulb at the centre of a curved reflecting mirror. The bulb gets its electricity from a battery that is larger than a shoebox. This provides enough power to make a very bright light. The reflector in the headlamp sends the light great distances in front of the car so the driver can see the road ahead. In normal conditions you can see the light leave the headlamp so the surface of the road shines but there is no light in the air. However something different happens when the car is driven in fog. Fog is made from billions of tiny drops of water. When light tries to shine through fog it is reflected off the surfaces of the drops and they light up. The water drops in the fog show where the light is travelling and one thing is very noticeable. The light does not spread out in all directions. It forms a beam with straight sides. If you shine your torch in mist or fog you will see the same thing. No matter where you shine your torch, a beam of light with straight sides will travel outwards from it into the water drops.

Torches are fun to use and you do not have to wait for a power cut or a foggy day to use them. Here is a simple experiment to try, if you want to see the beam of light your torch makes. Fill a glass with water, add a few drops of milk and stir them in. Shine your torch at the side of the glass and you will see a beam of light travel through the milky water. It does not matter how you shine your torch through the glass, whether you shine it upwards or downwards, a beam of light with straight sides will be made. Just try it.

Where does the electricity in our homes come from?
It comes from a power station, which may be many kilometres away. The electricity is made or generated in a power station. It is carried to our homes, to schools, hospitals and
factories on cables. Often the cables are carried over the ground on metal towers called pylons.

What happens in a power cut?
The electricity that is made at the power station is stopped from reaching your home. This may happen in bad weather. For example, gales may blow a cable off the pylon or so much snow and ice may settle on a cable that it is brought down. Sometimes a power cut is caused when repairs are being made to an underground electrical cable in a town.

What do the straight sides of a light beam tell us?
They show that light travels in straight lines. If it did not, the edges of the beams could be curved or wavy.

Are the straight edges due simply to the way the light is reflected in the torch?
No. Light travels in straight lines from any light source. You can test this in the following way. Take a comb and a piece of paper. Hold the comb so that it stands up on the paper and put the comb and paper near the bulb of a table light. You will see the small beams of light running across the paper from the gaps between the comb's teeth. The edges of these small beams are straight just like the beam from a torch. However, there is no reflector round a table lamp so the straight edges are not due to a reflector.

Is there a name for small beams of light?
Yes, they are called light rays. You can think of a beam of light as being made up of lots of light rays all travelling in the same direction.

What happens to the light rays coming out of a torch bulb?
Some light rays, which leave the bulb on the opposite side to the reflector, travel out from the torch in all directions. They make a weak circle of light when they fall on a surface. The light rays, which travel out from the bulb and hit the curved surface of the reflector, are all reflected so that they travel together. You can think of them as travelling in parallel lines. Those light rays at the edge of the beam give the beam its straight edge.

You can sometimes see beams of sunlight shining through a cloud. Why does this happen?
The light that makes the beam does not shine directly through a cloud. It shines through a gap in the clouds. The air below the cloud contains large amounts of dust. When the light shines through this air, some of the rays strike the dust and make the dust shine. This allows us to see where the light is travelling through the air.

Why are there not light beams under clouds?
The light rays that pass into a cloud from the Sun are scattered in all directions by the large water droplets and ice in the cloud. This makes the cloud appear white. The light rays, which leave the underside of a cloud, are travelling in all directions and do not make a beam.

You can sometimes see a beam of sunlight shine through curtains. Why does this happen?
There is dust in the air in the room. Some of the light rays in the beam strike the dust and make it shine. This allows us to see where the light is travelling through the air.
Why can't a beam of light be seen travelling through air all the time?
Air is transparent. Light rays pass straight through it. The beam can only be seen when there are particles in the air like dust or tiny drops of water. Some of the light rays in the beam strike the surfaces of these particles and make them shine.