Question

Give two reasons to explain why a reflecting telescope is preferred over a refracting telescope.

Hint:

Advantages of a reflecting telescope:
• No chromatic aberration.
• Large aperture possible.
• Greater light-gathering power.
• Easier and cheaper to manufacture large objectives.

Answer 1

Concept: A reflecting telescope uses a concave mirror as the objective instead of a lens. Reflecting telescopes are preferred over refracting telescopes because mirrors have several practical advantages over lenses. Reason 1: Absence of chromatic aberration.
A lens forms images by refraction and different colours of light suffer different amounts of refraction. Consequently, images formed by lenses suffer from chromatic aberration. A mirror forms images by reflection, and the law of reflection is independent of the wavelength of light. Therefore, \[ \boxed{ \text{Reflecting telescopes are free from chromatic aberration.} } \] Reason 2: Large aperture can be obtained easily.
In a refracting telescope, a large lens becomes very heavy and can be supported only at its edges, causing deformation and image defects. On the other hand, a mirror can be supported from its back, making it possible to manufacture mirrors of very large diameter. Therefore, \[ \boxed{ \text{Reflecting telescopes can have very large apertures and hence greater light-gathering power.} } \] Hence, a reflecting telescope is preferred because:
• It is free from chromatic aberration.
• Very large objective mirrors can be constructed, giving greater light-gathering and resolving power.