alleryStart learning →Why can a lens telescope not be built as large as a mirror telescope?
AA big lens cannot be polished
BA big lens absorbs all light
CA big lens sags under its weight
DA big lens must sit outdoors
Answer & Solution
Correct answer: C. A big lens sags under its weight
1. Light must pass right through a lens, so the lens can be held only at its rim.
2. Gravity then makes a large lens sag, which bends the light rays out of true.
3. The glass must also be flawless throughout and shaped precisely on both faces.
4. A mirror is supported from behind and only its front face must be shaped, so it can grow much larger.
_Source: OpenStax Astronomy (CC BY 4.0), section 6.1 Telescopes_
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