With these telescopes, light is either collected by a lens system as in the Museum's refracting telescope or by mirrors as in a reflecting telescope, or a combination of both. Refracting telescopes use lenses which are precisely curved pieces of glass that collect light. Lenses can be shaped so that all light rays passing through are bent to converge at a point called the focus, where an image is formed. The image is then magnified by the eyepiece and viewed by the observer.
Refraction occurs when light passes from one medium to another medium.
To give a concrete example, ask students to try this activity at home or at school. Put a pencil in a glass half filled with water. A refracting telescope - Note the light path and the inverted image. The focal length is the distance between the objective and the focus. The secondary mirror then reflects the light to a focus, a convenient l ocation for the image to be observed where it is then magnified by the eyepiece.