Consider the following statements about Light year: 1. Light year is a unit for measurement of very large distances. 2. Light year is a unit for measurement of very large time intervals. 3. Light year is a unit for measurement of intensity of light. Which of the statements given above is/are correct?
- (a)1, 2 and 3
- (b)2 and 3 only
- (c)1 and 2 only
- (d)1 only
Correct — D, 1 only. A light year is the distance light travels through vacuum in one year (about 9.46 × 10¹² km, i.e. roughly 9.46 trillion kilometres), so it measures very large distances — statement 1 is right. Despite the word 'year' in its name it is a unit of length, not of time, so statement 2 is wrong; and it says nothing about how bright a source is, so statement 3 (intensity) is wrong too. Only statement 1 stands, giving 1 only.
- (a)1, 2 and 3 — Statements 2 (time) and 3 (intensity) are both false — a light year measures distance only — so all three cannot be correct.
- (b)2 and 3 only — These are precisely the two false statements; a light year is neither a unit of time nor of light intensity.
- (c)1 and 2 only — Statement 1 is correct but statement 2 is false — the word 'year' misleads, yet a light year is still a distance, not a time.
A light year is an astronomical unit of distance — the distance a beam of light covers in vacuum in one year. Because the speed of light is huge (about 3 × 10⁸ m/s) but the gaps between stars are far larger, distances to stars are quoted in light years so the numbers stay manageable.
The trap is the word 'year', which tempts you to treat it as time. Read it as 'the distance light goes in a year' and both the time option and the intensity option fall away.
- One light year is about 9.46 × 10¹² km (roughly 9.46 trillion kilometres).
- Light travels at about 3 × 10⁸ m/s (nearly 3 lakh km per second) in vacuum.
- The nearest star beyond the Sun, Proxima Centauri, lies about 4.2 light years away.
- The parsec (about 3.26 light years) is another distance unit used by astronomers.
- Reading 'light year' as a unit of time because of the word 'year'.
- Treating a distance unit as a measure of brightness (intensity is measured in candela or lux).
Given as statements about what a light year measures — hold firm that it is a distance unit, so only the distance statement is correct.
UPSC_2021_GS1_Q762021Which one of the following is a reason why astronomical distances are measured in light-years?
- (a) Distances among stellar bodies do not change.
- (b) Gravity of stellar bodies does not change.
- (c) Light always travels in straight line.
- (d) Speed of light is always same.
Answer(d) Speed of light is always same.
Same concept — the light-year as a unit of distance. That UPSC item asks why astronomical distances are quoted in light-years (because the speed of light is a fixed constant); this NDA item tests that a light year measures distance, not time or intensity.
- practice — not a real PYQ
One light year is approximately equal to
- (a)9.46 × 10⁹ km
- (b)9.46 × 10¹² km
- (c)3 × 10⁸ km
- (d)365 km
Answer(b) 9.46 × 10¹² km — the distance light covers in one year in vacuum.
- practice — not a real PYQ
Which one of the following is a unit of distance?
- (a)Light year
- (b)Second
- (c)Candela
- (d)Pascal
Answer(a) Light year — it measures astronomical distance (second = time, candela = luminous intensity, pascal = pressure).