In a refracting telescope, what is the objective lens?

• A. A concave lens
• B. A plano-convex lens
• C. A convex lens that collects and focuses light
• D. A concave mirror

Answer: (C)

The main idea is that the objective lens in a refracting telescope must collect light from a distant object and form a real image by converging the incoming light. A convex lens does this because its outward-curving surfaces bend parallel rays toward each other, causing them to meet at the focal point and create a real image at the focal plane. That image is then magnified by the eyepiece for the observer.

If you used a concave lens, the rays would diverge instead of converging, so a real image wouldn’t form in the expected location. A plano-convex lens is still a convex lens with one flat side, so it can converge light, but the essential function of the objective is to collect and focus light, which a convex lens does. A concave mirror isn’t a lens at all and belongs to a different telescope type.