Lenses

The Starwaders Galilean Telescope Kit contains all the parts needed to build a small telescope in the same style as the one built by Galileo in 1609 – over 400 years ago.

This telescope is of the refracting telescope kind, by virtue of the fact that it uses lenses to ‘bend’ or refract the light to a focus point in order to accomplish the magnifying effect. Other methods of focussing light include reflecting the light off a curved surface. Telescopes that use that method are called reflector telescopes.

Lens and wavefronts
Lens convex

The Lenses

The lenses used in this kit are plastic lenses. The bigger lens at the front end is called the objective lens while the smaller one at the eye is called the eyepiece. The eyepiece is called a biconcave lens because each side (thus ‘bi’) is curved inwards (as in a cave). The objective lens is called a convex-concave lens because one side is convex (bulges outwards) while the other side is concave.

Light rays coming from very far away (like a star) are to all intents and purposes parallel to each other. The distance between the lens and the point where parallel light rays are brought to focus is called the focal length of the lens. When you use a magnifying glass to burn a leaf, you are actually focussing all the sunrays arriving at the surface of the magnifying glass into a very small point, which is actually a tiny image of the sun. If you hold the palm of your hand facing the Sun, you can feel the warmth. A magnifying glass is about the same size as your palm. All the heat energy in that area is being concentrated together, thus making the small focus area hot enough for the wood to start smouldering.

focal length

A Biconcave lens does not form a real image. Its focus point is on the same side of the lens as the where parallel rays are. This means that it cannot be used as a magnifying glass.

 

 


Page 3

Telescope Shop ScopeX Telescope SA Telescope