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 answer
Explanation
The correct answer is B, distance. A light year is the distance that light travels in empty space in one year, so despite the word "year" it measures length, not time.
Light moves at about 3 × 10⁸ metres per second. In one year it covers roughly 9.46 × 10¹² km, which is one light year. Astronomers use it because distances between stars are too large for kilometres. Proxima Centauri, the nearest star after the Sun, is about 4.24 light years away. Two other large units of distance are the astronomical unit (the mean Earth-Sun distance, about 1.5 × 10⁸ km) and the parsec (about 3.26 light years).
A is wrong: the unit includes a year only to define how far light goes; the quantity itself is a distance. C is wrong: the total light received by the Earth is measured as energy, not in light years. D is wrong: intensity of sunlight is measured in watts per square metre, as in the solar constant.
Exam tip: 1 light year ≈ 9.46 × 10¹² km; 1 parsec ≈ 3.26 light years; 1 AU ≈ 1.5 × 10⁸ km.
Practice Questions
View allA negative work is done when an applied force F and the corresponding displacement S are
- A.perpendicular to each other.
- B.parallel to each other.
- C.anti-parallel to each other.
- D.equal in magnitude.
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Work is negative when the force acts opposite to the displacement, because the angle between them is then 180°. Work done is W = F S cos θ, where θ is the angle between the force and the displacement.
Read the full explanation →Which one of the following statements about a satellite orbiting around the Earth is correct ?
- A.Satellite is kept in orbit by remote control from ground station.
- B.Satellite is kept in orbit by retro-rocket and solar energy keeps it moving around the Earth.
- C.Satellite requires energy from solar panels and solid fuels for orbiting.
- D.Satellite does not require any energy for orbiting.
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Once a satellite is placed in orbit with the right speed, the Earth's gravity keeps it going round without any engine.
Read the full explanation →Which one of the following is not a conservative force ?
- A.Frictional force
- B.Electric force
- C.Gravitational force
- D.Spring force
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Friction is a non-conservative force because the work it does depends on the path taken and the energy it removes is lost as heat.
Read the full explanation →A microphone converts
- A.electrical signals to sound waves
- B.sound waves to electrical signals
- C.microwaves to sound waves
- D.sound waves to microwaves
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A microphone is a transducer that changes the pressure variations of sound into a matching electrical signal. In a common dynamic microphone, sound waves make a thin diaphragm vibrate.
Read the full explanation →When a light ray passes through from air to water with a non-zero angle the ray will be
- A.bending towards the normal
- B.bending away from the normal
- C.propagating in straight line
- D.reflected towards the opposite direction
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Light slows down when it enters water from air, so a ray that strikes the surface at an angle bends towards the normal.
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