Scattering and soaking up light
Have you ever had to polish something? Perhaps it was cutlery, knives, forks and spoons or even a car. When you polish something you see it start to sparkle and reflect light more strongly. If you have done your polishing job well, you should be able to see reflections in the polished surfaces.

What happens to bring about this change? Imagine that you had to clean a dirty car. The surface of the car is covered in particles of mud and grit. If you were to run your fingers very gently over part of the car's body it would feel slightly rough to the touch due to these particles. When a light ray approaches a surface, it does so at a certain angle. If the surface is rough, the light ray that is reflected from it leaves at any angle. It could be the same angle as the approaching light ray but much more often it is not the same. This means that the light is scattered in every direction. Light that is scattered in this way does not produce reflections on the surface it is coming from. When you wash the car, mud and grit is removed. This makes the surface of the metal smoother and it may shine in the light. You may even see some reflections in it. However the surface of the metal is covered in tiny scratches and dents which cannot be seen. These form from particles of dirt and grit in the air, which collide with the car as it moves along. When you polish the metal, you fill in these cavities and make the surface even smoother. Light rays which strike a smooth surface leave it at the same angle as they approached it. If all the light rays are travelling together in parallel from your face they will leave in parallel from the car's surface and appear to come from an image of your face in the metal.

If you gently feel the surface of a mirror you will find that it is very smooth. If you touch other objects such as paper or your clothes you will feel that they are rough. In fact, the surfaces of most objects are rough and scatter light just like the dirty car - in all directions. This means that no images form in the surfaces of these materials and so, when you go into a living room, you do not see images of yourself in the chairs, carpets or curtains.

When light strikes any object, something else happens in addition to reflection. Some of the light is soaked up or absorbed by the object. If an object soaks up all the light that strikes it, none is reflected. If we cannot see any light coming from a surface, that surface appears black. Some surfaces absorb only a small amount of light. Most of the light that strikes them is reflected and makes the surface appear white.

However most surfaces are neither black nor white but have a colour. Where do the colours come from? They come from the light itself.

The discovery that a ray of light is actually made from a mixture of light of different colours was made in 1666 by Isaac Newton. He darkened a room then made a small circular hole in a shutter. He placed a triangular glass block near the hole and let the light shine through it. The light crossed the room and fell upon a screen. It did not make a white spot but a series of coloured bands. The bands were red, orange, yellow, green, blue, indigo and violet.

When light falls on most objects, some of the colours of light are absorbed. The colours that are not absorbed are reflected and give the object its colour. For example, if sunlight falls on a leaf, all the colours except green are absorbed. The green light is reflected and we see the leaf as green. In a similar way, a combination of colours is reflected from the wood on the twig to make the wood appear brown.

As you look around you now, think of the amounts of light being absorbed and being scattered. Which objects are absorbing all the light that strikes them so they appear black? Which objects scatter most of the light that strikes them and so appear white? Which objects scatter just blue, yellow or red light? You may find yourself looking at your surroundings in a whole new way.

How does polish make a surface smooth?
Polish is a wax. When you smooth it onto a surface it fills all the scratches and dents. This makes the surface smoother so it can reflect light better.

How is a reflection made?
When light from an object strikes a smooth surface all the rays are reflected back in the same direction. This makes them seem to appear to be coming from an object behind the reflecting surface. The reflection of an object in a smooth surface, such as a mirror, is called an image. The image of the object seems to be the same distance behind the reflecting surface as the distance between the object and the reflecting surface.

If you take a piece of shiny metal and rub it with sandpaper you can no longer see an image in it. Why is this?
The shiny metal has a smooth surface and rays of light, which reach it from an object, are all reflected in the same direction and seem to be coming from an image in the metal. When you rub sandpaper on the metal, the surface is made rougher. The rays of light, which strike the surface now from an object, are reflected in all directions. They do not seem to appear to come from an image and so no reflection is seen in the surface.

Some yellow jackets worn by the police or by people working on the roads reflect light. How do they do it?
The reflecting materials have a layer of tiny glass beads in them. When light strikes the beads it is scattered inside the material then reflected back to make the jackets shine.

How do reflectors on cars and bicycles work?
The reflectors have surfaces set at a certain angle. When light strikes them from the headlamp of a following vehicle, the light is reflected up to that driver so the reflector can be easily seen.

How does a glass prism split up light?
A ray of light is made from a mixture of light of different colours. All light travels as waves of energy. You can think of them as moving like the waves called ripples, which move across a puddle when you tap its edge. One property of a wave is its wavelength. This is the distance between two wave crests. The different colours of light have wavelengths of different lengths. Red light has the longest wavelength and violet has the shortest. When the light is travelling through air all the different coloured light waves move along at the same speed. However, when light passes through a prism, it slows down. When light is slowed down like this, its path is bent. This process of bending is called refraction. Light with longer wavelengths is refracted more than light with shorter wavelengths. This means that as the different coloured light waves have different wavelengths, they are refracted by different amounts so that when they leave the prism they have become separate and make a spectrum of seven colours.

How is a rainbow made?
You can sometimes see a rainbow if you stand with your back to the Sun when it is raining. You can also see a rainbow in water coming from a hose or fountain if you have your back to the Sun. When the sunlight passes you and enters the drops of water it is refracted. This makes the seven colours of light pass through the raindrop and strike the other side. When this happens, the light is reflected though the water drop to near where the light entered. As the seven colours of light pass out of the water drop they are reflected again. These processes of refraction and reflection, occurring in millions of raindrops at the same time, produce the colours you see in a rainbow.

Why is the sky blue?
All the colours of light travel together across space from the Sun until they reach the Earth's atmosphere. The Earth's atmosphere is made up of billions of particles of different gases. When the rays of white light reach the atmosphere the blue light is scattered by the particles in the atmosphere and this makes the sky seem blue.

Why does the Sun change colour when it is near the horizon?
The horizon is the place in the distance where the sky meets the surface of the Earth. When the Sun is high in the sky, it shines with a white yellow light. Remember you must NEVER look directly at the Sun, as it will damage your eyes. When the Sun is low in the sky, near the horizon, it shines with a red orange light. This difference is due to the atmosphere. In addition to gas particles the atmosphere also contains huge numbers of dust and smoke particles. These also scatter light. When the Sun is overhead, it shines directly down through the atmosphere but when it is on the horizon it shines through the atmosphere at an angle. This makes the light shine through more of the atmosphere and more light is scattered by the dirt and smoke so that only red and orange light are left to shine directly through and give the Sun its colour.