Ray Diagrams

Telescopes work by collecting a lot of light and using mirrors (Reflecting telescopes) or lenses (Refractors) to focus the light, i.e. to bring it together at a point.

We can use ray diagrams to show how the light goes through the lenses and reflects off the mirrors inside the telescope.

The example ray diagram on the left shows three imaginary "rays" of light being reflected off a mirror and then going through a lens.

You can see where the focus is by looking where the "rays" cross each other.

It is the lens that is making the light focus. If you want to use a mirror to focus light, then it needs to have a curved surface. It is these curved mirrors that are used in telescopes.

Here are some ray diagrams for several different types of telescope:

Telescopes with Lenses

• Refracting Telescope

Telescopes with Mirrors

• Prime Focus Telescope
• Newtonian Focus Telescope
• Cassegrain Focus Telescope