Where sunlight reaches
The Sun is a star. Like almost all stars it gives out light. The light, which shines out of the Sun's surface, travels in straight lines. These lines of light are called light rays. As the Sun is globe-shaped, the light rays that leave its surface travel outwards in all directions. Most of them shine out into space for billions of kilometres. If you were at such a great distance away, the Sun would look like most other stars that we see in our sky. But we are much closer so the Sun appears much larger. Around the Sun are nine planets. Let us see what happens to rays of sunlight when they reach some of them.

The planet that is nearest to the Sun is Mercury. It is fifty eight million kilometres from the Sun, nearly three times closer to the Sun than the Earth is. This means that the Sun appears much larger in the sky from Mercury than it does on Earth and is much brighter. There are no clouds in the atmosphere on Mercury so all the light rays reach the planet's surface. In the past large numbers of rocks have rained down on Mercury's surface and made craters. The light rays reach down into the bottom of the craters and make them shine. Around the rim of a crater is a high wall of rock. Some of the rays strike one side of the wall but they cannot shine through the rock and reach the other side. This means that sunlight fails to reach the other side of the wall. A place which has no light is in the dark, so the other side of the wall is dark and casts a shadow on the planet's surface.

Some of the Sun's rays do not strike Mercury. They pass it by and strike the next planet to the Sun. This planet is Venus and it is one hundred and eight million kilometres from the Sun. In the atmosphere of Venus are very thick clouds. They cover the whole planet. When many of the light rays strike the cloud tops they are reflected back into space. This makes the surface of the planet appear white. However, some light rays enter the cloud. They do not pass straight through as sunbeams but are split up and scattered in all directions. When the light rays come out beneath the cloud they travel to the planet's surface. It is covered with lava and the remains of volcanoes. The light rays strike the surface from all directions so strong shadows are not made. As only a small amount of light reaches below the clouds the sky is very gloomy ? like we see on Earth when black clouds pass overhead.

Some of the Sun's rays that pass Mercury and Venus reach the Earth. There are some clouds in the Earth's atmosphere. Light rays which strike the tops of the clouds are reflected back into space. Light rays which pass through the clouds are scattered in all directions. However there are plenty of places in the atmosphere where clouds are not present and the Sun's rays can shine straight down onto the surface like the Sun's rays that shine on Mercury. Much of the land on the Earth is covered with plants such as trees and bushes. When the light rays shine on a tree they light up its surface so we can see it. The light rays cannot pass through the wood in the tree so on the other side there is darkness, which we call shade. There is also a black outline of the tree on the ground, which we call the tree's shadow.

You may think that all the light that the Sun makes shines on the Earth, but most does not. Sunlight passes the Earth to shine on the sands and rocks, which cover the surface of Mars, the next planet to the Sun after the Earth. Other rays shine on the cloud tops of the more distant planets of Jupiter, Saturn, Uranus and Neptune. The most distant planet from the Sun is Pluto. It is so far away that the Sun looks almost like any other star in the sky. Only a few light rays reach Pluto so the sky and planet surface are always dark.
Back on Earth just look out of a window. If it is daylight what are the light rays doing? Are they scattered by a cloudy sky and so do not form shadows? Are they coming straight in from the Sun in a cloudless sky and making everything bright and causing dark shadows to form? Have a look.

Is sunlight like firelight?
No, but some people used to think that the Sun burnt like a piece of coal in space. However, for something to burn, air is needed and there is no air in space. The light and heat in the Sun are not produced in the same way as the light and heat from a burning fire. They are produced by changing one substance into another. The Sun is made from two gases called hydrogen and helium. Helium gas is used to make party balloons float in the air. Hydrogen is similar to helium but easily catches fire when mixed with air. In the Sun the gases are squashed together by the force of gravity. This is the force that we have on Earth too. It makes things fall. In the Sun the force of gravity is much stronger and at the centre of the Sun it squashes hydrogen until it changes into helium. In this change light and heat are made.

Is it dangerous to look at the Sun?
Yes. Very dangerous. It can damage your eyes and make you blind. You must NEVER look directly at the Sun.

Are all the other stars just like the Sun?
No, some are smaller and some are larger. The Sun shines with a yellow light. Some stars shine with a red, blue or white light.

Which stars do not shine?
Those stars which have used up all the hydrogen. If hydrogen is not being turned into helium, light is not being made. A star which no longer shines is called a black dwarf star.

Why are there shady places?
A shady place is one where light rays shining straight from the Sun cannot reach. Think of a tree in the sunshine. The sunlight shines down on its branches and leaves. The light rays cannot pass through the wood and the leaves. Their paths are blocked. This makes the place below the tree dark.

Why is it not completely dark under the tree?
When light rays strike a surface some of them are reflected. That means they bounce off the surface in a different direction. Imagine that there was an ice cream van near the tree. Some of the sunlight striking the side of the ice cream van would be bounced off into the area under the tree. This light prevents the area under the tree from being completely dark.

Where could you go to get away from sunlight on a sunny day?
Simply go inside and shut the curtains. If there are no gaps between the curtains and the wall and the curtains are made of thick materials which blocks all the light rays, the room will be completely dark. You will have shut out all the Sun's rays.

Is a shadow like a reflection?
No. A reflection is made by light bouncing off a shiny surface. For example, you see a reflection of yourself in the mirror because the light from your face bounces off the shiny surface into your eyes. A shadow is a place where light is absent. It is caused by light rays being blocked. You can see details in a reflection such as the eyes on a face. When you look at a shadow of a head you cannot see any details such as eyes.

What is a silhouette?
It is the black outline of an object when seen against the light. If you look at the horizon just after sunset you will see that there is light in the sky but the objects on the horizon such as trees and houses only appear as black outlines. These are silhouettes.