How a telescope works

A telescope is an instrument used to collect electromagnetic radiation to view and magnify distant objects. It collects electromagnetic radiation such as light, infrared or radio waves.

A simple light telescope can be made using just two convex lenses.

View the video on the right and answer the questions below.

1) A simple telescope is made of two lenses a thick lens and a thin lens bright lens and a dark lens round lens and an oval lens.

2) A thick lens weighs more than a thin lens bends the light more than a thin lens is less round than a thin lens. allows less light to pass through than a thin lens.

3) A powerful lens is said to be bigger than a less powerful lens. allow more light through than a less powerful lens. bend the light more than a less powerful lens.

4) A powerful lens creates an image that is bigger but darker than a less powerful lens. larger than a less powerful lens. smaller and is clear than an image produced by a less powerful lens

5) A telescope is made of two lenses both thick powerful lenses. both thin lenses. a thick lens and a thin lens.

6) The lens that is placed at the front of the telescope to allow light to pass through is a thin lens that creates a bigger image than a thick lens. a thick lens that creates a bigger image than a thin lens a thick lens that magnifies the image more than a thin lens.

7) The eyepiece of a telescope is made of a thick lens so that the image produced by the thin lens appears brighter. is made of a thick lens that magnifies the image produced by the thin lens. is not a lens but a piece of glass.

But just how does each lens create an image of a distant object?

We shall use ray diagrams to explain this.

Lets look at how the first lens bends the light to form an image of the arrow.
Click to see the image. We can draw the image using two lines.
First draw a line that runs parallel from the tip of the arrow to the lens and through the focal point.
Next draw a line that passes from the tip of the arrow through the lens.

Where the two lines meet is the tip of the arrow of the image.

The eyepiece is now moved so the image formed by the first lens is inside its focal length. Once again we draw the two lines but you will notice that they do not meet. An observer will see light passing through the eyepiece that is spreading out.
Click to see the ray diagram.

The brain now constructs a magnified image because it assumes that the light has come from a single point behind the lens. A sit track the light back to where the light rays appear to be coming from a virtual, magnified image is produced.

Click to see the ray diagram.

Magnified images are virtual images. That is, light does not actually focus on a point in order to form this image. Every time you bring an object inside the focal length of a magnifying glass a magnified image is produced. The further away from the object we place the lens the greater the magnification, as shown on thr right, until the image becomes blurred. This happens when the object is exactly one focal length away from the lens.

Using ray diagrams explain why the image is blurred when the object is at the focal point.

View the video to see how to construct your Galileon telescope with the equipment provided.

Exercises for ray diagrams.