Ray Diagrams

Telescopes collect light from distant objects in space. They bring the light together at a focus. The focused image is viewed using the eye. Or the light can be measured using an instrument.

Not all telescopes focus light in the same way. Refracting telescopes use lenses. Reflecting telescope use mirrors.

We can use ray diagrams to compare types of telescope. These show the path that light takes.

Example Ray Diagram
Credit: NSO

The ray diagram in the picture shows 3 'rays' of light entering the telescope. Each ray hits a mirror and is reflected off at 90 degrees. The 3 rays then pass through a glass lens. The light is refracted by the lens. Each ray exits the lens at a different angle. Together the mirror and lens 'steer' the rays so that they all meet at a chosen focus point. The focus is where the rays cross each other. A mirror can be used in place of the lens, but it must have a curved surface.