Light year is a measure of
- (a)time
- (b)distance
- (c)total amount of light falling on the Earth in a year
- (d)average intensity of light falling on the Earth in a year
Correct — B, distance. A light year is the distance that light travels through vacuum in one year. Multiply the speed of light, about 3 lakh kilometres every second, by the number of seconds in a year and the product is a length of roughly 9.46 million million kilometres. The word year sits inside the name only because a year is the interval used in the multiplication, in the same way that a kilometre per hour contains the word hour without being a measure of time. Astronomers use it because the speed of light in vacuum is a fixed constant, so the unit means the same thing everywhere.
- (a)time — This is the trap the question is built on, and it comes from reading only the second word of the name. The unit is speed multiplied by time, and speed times time is a length.
- (c)total amount of light falling on the Earth in a year — That would be a quantity of radiant energy, measured in joules. A light year says nothing about how much light arrives anywhere.
- (d)average intensity of light falling on the Earth in a year — Intensity is power per unit area, measured in watts per square metre. It is a different physical quantity altogether and has no connection with the light year.
Astronomical distances are far too large for kilometres to be readable, so astronomers use units built from natural yardsticks. The astronomical unit is the mean distance from the Earth to the Sun, about 149.6 million kilometres, and suits distances within the solar system. The light year and the parsec suit distances between stars; one parsec is about 2 lakh astronomical units, or roughly 3.26 light years. All three are lengths, not times.
A light year carries a second idea worth keeping: because light takes time to arrive, looking far away is also looking back in time. A star four light years off is seen as it was four years ago. That is why observing very distant galaxies is a way of studying the young universe. In examinations the light year usually appears either as this straight definition or inside a statement set that plants the false claim that it measures very large time intervals.
- One light year is the distance light travels in vacuum in one year, about 9.46 million million kilometres.
- The speed of light in vacuum is about 3 lakh kilometres per second and is the same everywhere, which is why the unit is dependable.
- One astronomical unit is the mean Earth-to-Sun distance, about 149.6 million kilometres.
- One parsec is roughly 3.26 light years, or about 2 lakh astronomical units.
- Because light takes time to travel, an object seen at a distance of n light years is seen as it was n years ago.
- Reading the word year and answering time; a light year is a length.
- Confusing the light year with the astronomical unit, which is a solar-system yardstick.
- Assuming a parsec is smaller than a light year — it is about 3.26 times larger.
NDA asks for the quantity a named unit measures, or sets two or three statements about the light year and asks which hold.
Which 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.
Asks why the unit works rather than what it measures — the constancy of the speed of light is what turns a duration into a fixed length.
Light year is a unit for measurement of
- (a) age of universe
- (b) very small time intervals
- (c) very high temperature
- (d) very large distance
Answer(d) very large distance
The same item a year later with the options reshuffled, which is a good measure of how reliably NDA repeats this definition.
- practice — not a real PYQ
One astronomical unit is the average distance between
- (a)the Earth and the Moon
- (b)the Earth and the Sun
- (c)the Sun and Jupiter
- (d)two neighbouring stars
Answer(b) the Earth and the Sun — about 149.6 million kilometres, the yardstick used inside the solar system.
- practice — not a real PYQ
A star is said to be 8 light years away. This means that
- (a)the star is 8 years old
- (b)light from the star takes 8 years to reach us
- (c)the star will disappear in 8 years
- (d)the star gives out light for 8 years at a time
Answer(b) light from the star takes 8 years to reach us — the distance is what light covers in that time, so we see the star as it was 8 years ago.