Which one of the following telescopes contains only mirrors?

Correct — C, Newtonian telescope. A Newtonian reflecting telescope uses only mirrors — a concave primary mirror and a small flat secondary — to gather and focus light, so it contains no objective lens.

Why the others are wrong

• (a)
  Galilean telescope — A refractor: it uses a convex objective lens and a concave eyepiece lens, not mirrors.
• (b)
  Keplerian telescope — Also a refractor, using two convex lenses (objective and eyepiece).
• (d)
  Schmidt telescope — A catadioptric design that combines a mirror with a corrector lens (Schmidt corrector plate), so it is not mirrors only.

Telescopes fall into refractors (lenses), reflectors (mirrors) and catadioptric (both). The Newtonian, invented by Isaac Newton, is a pure reflector using a parabolic primary mirror and a flat diagonal secondary — avoiding the chromatic aberration of lens telescopes.

Key facts

• A Newtonian telescope is a reflector using only mirrors.
• It has a concave primary and a flat secondary mirror.
• Galilean and Keplerian telescopes are refractors (lenses).
• Schmidt telescopes are catadioptric (mirror + corrector lens).

Common traps

• Assuming any telescope uses lenses — reflectors use mirrors.
• Thinking the Schmidt is mirrors-only — it also has a corrector lens.

• practice — not a real PYQ

  A refracting telescope uses which optical component to gather light?

  • (a)a concave mirror
  • (b)an objective lens
  • (c)a diffraction grating
  • (d)a prism
  Answer(b) an objective lens.
• practice — not a real PYQ

  Reflecting telescopes are preferred for large apertures mainly because they avoid

  • (a)gravity
  • (b)chromatic aberration
  • (c)reflection
  • (d)magnification
  Answer(b) chromatic aberration.