How many lenses are in a refracting telescope?

• A. Two
• B. One
• C. Three
• D. Four

Answer: (A)

Lenses in a refracting telescope work together to collect light from a distant object and then enlarge that image for the eye. The front lens, the objective, gathers light and forms a real image at its focal plane. The second lens, the eyepiece, sits near the eye and acts as a magnifier for that image, producing a larger view. With only one lens, you’d either magnify a nearby object or fail to create the enlarged view of a distant object that a telescope provides. So, two lenses are used: one to form the image and one to magnify it, with magnification set by the focal lengths of the two lenses.