Try this: Make a telescope

You will need

• Two magnifying lenses, one more powerful than the other.

What to do

1. Close one eye.
2. Hold the more powerful lens in front of your open eye.
3. With the other hand, hold the weaker lens behind the stronger one (ie further away from you).
4. Move the weaker lens away from you. Keep it in the middle of your field of view. This may be tricky, as your view will be blurred from looking through the more powerful lens.
5. When the weaker lens is the right distance away, you will see distant objects come into focus. They will appear much larger, but also upside down.
6. Try looking around at different things through this telescope. You will find you need to adjust the positions of the lenses to look at things at different distances.

Caution: Do not look at bright objects with your telescope, the intense light can hurt the light sensors in your eye and damage your vision. Do not look at the Sun under any circumstances.

What's happening?

You have made a simple telescope.

You have probably noticed that if you hold a lens near a wall opposite a window, you can project an image of the outside world onto the wall. Our telescope works in a similar way, but uses the effect twice. First, the light coming through the weaker lens (called the objective lens) is focused, the same way it would be if you were to focus it against a wall. Since there is no wall there, the light continues through until it reaches the second lens (called the eyepiece). This lens acts as a magnifying glass and, when combined with the lens in our eye, focuses the light onto the light sensors on the back of our eyes, so we can see it.

Normally, when you look at a distant object, the angle between the light reaching you from the bottom of the object and the light from the top of the object is pretty small. When you look through the telescope, this angle is increased, so the object looks larger. (There is a diagram of this on the web site).

The path the light takes to reach your eyes normally (top) and through our telescope (bottom). Note the difference in the angle the light makes when it reaches your eye. This is why objects appear larger.

Telescopes have two main advantages over looking at stars and planets with the naked eye:

• They make the thing you are looking at appear larger. This is useful for looking at the moon and planets, but the stars are so far away that no telescope is powerful enough to make them look bigger than dots.
• They collect more light than our eyes. The pupil of my eye (the black part where the light goes in) is never more than about 7mm wide. Any light that hits the pupil of my eye is focussed on the back of my eye so I can see things. A circle 7cm wide has 100 times the area of a circle 7mm wide, so a 7cm wide telescope will gather 100 times as much light as my eye. This makes stars appear much brighter than looking at them with the naked eye.

Related Activities

• How lenses work

Applications

• Astronomers have used telescopes for centuries, to observe the planets and stars. The type of telescope we made is called a keplerian telescope, after the astronomer Johannes Kepler. It is a type of refracting telescope, which works by bending light with a lens. Many telescopes used curved mirrors instead of lenses, these are called reflecting telescopes. Most large telescopes (such as the Hubble Space Telescope) are reflectors, because it is easier to make a large mirror than a large lens.
• The first person to see the rings of Saturn was Galileo Galilei, in 1610. He used a telescope similar to ours, but his telescope had a diverging lens instead of a magnifying lens for an eyepiece.
• If you reverse your lenses, so the more powerful lens is away from you, you can make a simple microscope.