Shadows and silhouettes
Every few years an unusual event takes place in space. The Moon passes between the Sun and the Earth. When this happens, light coming from the Sun, on its way to the Earth, is stopped by the surface of the Moon. The light cannot pass through the Moon because the Moon is opaque. This means that on the side facing the Earth there is no light and a shadow is cast upon the surface of the Earth. People living in places where the shadow is cast see an eclipse of the Sun.

This phenomenon occurs because rays of light travel in straight lines. If they did not do this something strange would take place when the Sun, Moon and Earth were all in line. The light rays, which pass very close to the Moon but carry straight on and reach the Earth, would behave in another way. They would bend round onto the side facing us and perhaps strike the mountaintops on the Moon. The light from the mountaintops would be reflected to Earth. Other rays might skim along the surface of the Moon and strike it causing more reflections so the surface of the Moon would be seen on Earth during an eclipse.

It is not safe to look directly at the Sun and Moon during an eclipse. The best way to see them is to let the light from them shine through a small telescope or one half of a binocular onto a sheet of white paper. You then look at the sheet of white paper to see the outline of the Moon as it crosses in front of the Sun. The black outline of the Moon is called the silhouette.

You do not have to wait until you see shadows and silhouettes. Just look down on the table in front of this computer screen. If there is a light on in the room, there is a chance you may see the shadow of the keyboard, mouse or even yourself. All these shadows are made because light is stopped by opaque objects. If you pick up a pen and hold it at arm's length towards the light it may appear as a black line. If this happens, you are seeing the silhouette of the pen.

There are two kinds of light sources and they affect the way shadows are made. The first kind of light source is tiny. It is called a point source. You can make a point source of light by putting a pinhole in a piece of dark card and setting up the card in front of a light bulb. When you switch on the bulb, some light rays shine out through the pinhole. If you put an object near the pinhole and a sheet of paper behind the object you see a dark shadow with sharp edges. However, if you remove the card and let light from the whole surface of the bulb shine on the object something different happens. The shadow has fuzzy edges and the outer parts of the shadow are grey while a small inner part remains black. This is due to the change in the source of light.

Once the card has been removed from the lamp, light rays from all parts of the bulb can shine on the object. Those that shine from the centre of the bulb are stopped by the object and produce the black central part of the shadow. This part of the shadow is called the umbra. Light rays from the top and bottom of the bulb strike the surface of the object and cast the outer part of the shadow. However, light from other parts of the lamp, from between the top and bottom and the centre, also shines into the shadow made by the light from the top and bottom of the lamp. This light makes the outer part of the object's shadow greyer with fuzzy edges. This part of an object's shadow is called the penumbra.

Most of the light sources we use are large light sources like the lamp without the card, which has just been described. In fact some bulbs are designed to spread out light. For example, a light bulb with clear glass lets light from the filament pass straight through the glass because the glass is transparent. The filament behaves like a point source of light and produces dark shadows with sharp edges. However, many light bulbs have a pearl surface. This is translucent and scatters the light from the filament in all directions. This scattering of the light makes the light bulb into a large light source and the shadows it produces are greyer and have softer edges. Fluorescent lamps are also designed to produce these gentler shadows. The reason for producing shadows of this kind is that people find that they are more restful and things are easier to see using large light sources.

Even with large light sources it is possible to make dark shadows with sharp edges and shadows which show an umbra and penumbra. Just place a pencil on the desktop near your keyboard and look for its black shadow. Now slowly raise the pencil and see how its shadow changes.

Do only opaque objects make shadows?
No. Opaque objects make the darkest shadows. An object made of a translucent material makes a greyer shadow because some of the light is stopped but some is allowed to pass through. A transparent object makes a faint shadow because some of the light is reflected from its surface. This reduces the power of the light passing through the object, so the light which leaves the other side of the object casts a faint grey shadow.

Why are some materials transparent while others are opaque?
Light is a form of energy. It travels in the form of waves. These waves behave in some ways like electricity and in other ways like a magnet. They have electrical and magnetic properties and are called electromagnetic waves. All materials are made of particles and in some materials the particles are arranged so that the waves can pass through the material. These materials are transparent. Most materials have particles which are arranged in such a way that they stop the movement of these waves. These materials are called opaque materials.

What happens to the light when it strikes an opaque material?
Some of it is reflected. We see all opaque objects by the light that they reflect. Some of the light is absorbed by the material.

Can an object have more than one shadow?
Yes, it can. Each light source shining on it makes the object cast a shadow on the opposite side from the light source. If your computer is set up near a window and there is a light on in the room, hold up a pencil just below the computer screen and you should see two shadows. One is from the light from the window and one is from the light in the room.

Are shadows only made by the light from light sources?
No. If you go out on an evening of a full Moon, you will see that the light from the Moon casts shadows. The Moon is not a luminous object. It reflects light from the Sun. If you shine light from a lamp onto a mirror and put an object and sheet of paper near the mirror, you will see the object cast a shadow from the reflected light.

Why do shadows move round outside during the day?
They move round because the position of the Sun changes. As the Sun rises in the east, long shadows are cast pointing towards the west. During the morning the Sun rises to its highest position in the sky and the shadows become shorter and point towards the north in the Northern Hemisphere. At midday the Sun is at its highest position and the shadows are at their shortest and point north. In the afternoon, the Sun sinks towards the west. The shadows lengthen again and point towards the east.