Why are major optical observatories usually built on mountains?

Thinner, steadier, drier air above them

Radio telescopes detect what?

Radio waves emitted by objects in space

Why are some telescopes placed in orbit?

The atmosphere blocks and blurs much of the spectrum

Adaptive optics improves ground-based images by doing what?

Deforming a mirror many times a second to cancel atmospheric blur

Interferometry combines several telescopes to achieve what?

The resolving power of a much larger instrument

A light-year of distance means we see an object how?

As it was when the light left it

Slightly ahead of its current state

Spectroscopy allows astronomers to determine what?

Composition, temperature and motion of distant objects

The largest optical telescopes today use mirrors built how?

From many hexagonal segments acting as one

From a single cast piece of glass

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Space & Astronomy Medium ⚡ 10 Questions ⏱️ ~7 Mins +100 XP

Telescopes and How We Look

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Most of what we know about the universe arrived through instruments, and the instruments shape what we can know.

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About this topic

Astronomy is limited by what can be detected. For most of its history that meant visible light, which is a narrow slice of what the universe emits, and every extension into another part of the spectrum has produced discoveries nobody predicted. These questions cover how telescopes work, why they are placed where they are, and what different instruments are actually detecting — including several that collect no light at all.

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Telescopes and How We Look: all 10 questions and answers

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1A refracting telescope gathers light using what?
- A A prism only
- B Lenses Correct
- C Mirrors
- D A radio dish
Answer: B. Lenses

Why: Reflectors use mirrors instead, which is what almost all large modern telescopes do because big mirrors are easier to support than big lenses.
2Why are major optical observatories usually built on mountains?
- A Thinner, steadier, drier air above them Correct
- B Lower gravity at altitude
- C Closer to the stars
- D Cheaper land
Answer: A. Thinner, steadier, drier air above them

Why: Atmospheric turbulence blurs images, so height, dry air and distance from city lights all help. Altitude gets you above a good share of the water vapour.
3Radio telescopes detect what?
- A Sound travelling through space
- B Reflected sunlight only
- C Cosmic dust particles
- D Radio waves emitted by objects in space Correct
Answer: D. Radio waves emitted by objects in space

Why: They can operate in daylight and through cloud, and they revealed pulsars and the cosmic microwave background.
4Why are some telescopes placed in orbit?
- A To avoid light from the Moon
- B To reduce their cost
- C The atmosphere blocks and blurs much of the spectrum Correct
- D To be closer to their targets
Answer: C. The atmosphere blocks and blurs much of the spectrum

Why: X-rays and much infrared never reach the ground at all, so those instruments have to be above the atmosphere to work.
5Neutrino detectors are typically built where?
- A On the sea surface
- B Deep underground or under ice Correct
- C On mountain summits
- D In low orbit
Answer: B. Deep underground or under ice

Why: Rock or ice filters out other particles, leaving neutrinos, which pass through almost everything and interact very rarely.
6Adaptive optics improves ground-based images by doing what?
- A Deforming a mirror many times a second to cancel atmospheric blur Correct
- B Increasing exposure time
- C Cooling the lens
- D Filtering out blue light
Answer: A. Deforming a mirror many times a second to cancel atmospheric blur

Why: A reference star, sometimes an artificial one created with a laser, is used to measure the distortion in real time.
7Interferometry combines several telescopes to achieve what?
- A A brighter image only
- B Wider colour range
- C Faster exposures
- D The resolving power of a much larger instrument Correct
Answer: D. The resolving power of a much larger instrument

Why: Linking dishes across continents is how the first image of a black hole's shadow was produced.
8A light-year of distance means we see an object how?
- A Slightly ahead of its current state
- B Only in outline
- C As it was when the light left it Correct
- D As it is right now
Answer: C. As it was when the light left it

Why: Looking further away is therefore looking further back in time, which is how the early universe can be observed at all.
9Spectroscopy allows astronomers to determine what?
- A The age of a telescope
- B Composition, temperature and motion of distant objects Correct
- C Distance alone
- D Only apparent brightness
Answer: B. Composition, temperature and motion of distant objects

Why: Splitting light reveals absorption lines, and their shift toward red or blue shows whether an object is moving away or toward us.
10The largest optical telescopes today use mirrors built how?
- A From many hexagonal segments acting as one Correct
- B From a single cast piece of glass
- C From polished metal sheets
- D From liquid mercury only
Answer: A. From many hexagonal segments acting as one

Why: Segments can be made, transported and aligned individually. A single mirror beyond about eight metres begins to sag under its own weight.

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