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.
Correct answer
Explanation
The correct answer is D, Speed of light is always same.. A light-year works as a unit of distance only because light in a vacuum always travels at the same speed, so the distance it covers in a year is fixed. That speed is about 3 lakh kilometres per second, and in one year light covers roughly 9.46 trillion kilometres, which is one light-year. Distances between stars and galaxies are so huge that kilometres become unwieldy: the nearest star after the Sun, Proxima Centauri, is about 4.24 light-years away. Since the speed of light is a universal constant, the unit gives the same value everywhere and at all times. Option A is wrong because stars and galaxies move, and the distances between them keep changing; the universe itself is expanding. Option B is wrong because gravity has nothing to do with defining a unit of length, and the gravity of stars can change as they evolve. Option C is wrong because light bends near massive bodies, an effect called gravitational lensing, and in any case the straight path is not the reason for the unit. Exam tip: a light-year is a unit of distance, not time; 1 light-year is about 9.46 × 10^12 km.
Practice Questions
View allWhich one of the following is not used as fertilizer?
- A.Ammonium nitrate
- B.Ammonium sulphide
- C.Ammonium phosphate
- D.Ammonium sulphate
Show answer
Ammonium sulphide is an unstable, foul-smelling compound that breaks down to release ammonia and toxic hydrogen sulphide, so it is not used as a fertiliser.
Read the full explanation →The focal length of the objective lens of a telescope is 50 cm. If the magnification of the telescope is 25, then the focal length of the eye-piece is
- A.12.5 cm
- B.5 cm
- C.2 cm
- D.10 cm
Show answer
For an astronomical telescope in normal adjustment the magnifying power equals the focal length of the objective divided by that of the eyepiece, so the eyepiece focal length is 50 ÷ 25 = 2 cm.
Read the full explanation →The major source of vitamins and minerals for vegetarians is
- A.black gram and wheat.
- B.rice and mustard.
- C.vegetables and fruits.
- D.soya bean and milk.
Show answer
Vegetables and fruits supply most of the vitamins and minerals in a vegetarian diet, which is why they are called protective foods.
Read the full explanation →Which one of the following compounds is used in black and white photography?
- A.AgF
- B.AgBr
- C.AgCl
- D.Ag2SO4
Show answer
Silver bromide is the light-sensitive compound coated on black and white photographic film and paper, because light breaks it down into tiny grains of silver metal.
Read the full explanation →Which one of the following statements about speed and velocity is correct?
- A.Speed and velocity both are vector quantities.
- B.Speed and velocity both are scalar quantities.
- C.Speed is vector quantity and velocity is scalar quantity.
- D.Speed is scalar quantity and velocity is vector quantity.
Show answer
Speed tells only how fast a body moves, while velocity also tells the direction of motion, and a quantity with both magnitude and direction is a vector.
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