Twinkling of stars is due to
- A.particular frequencies of the starlight.
- B.reflection of starlight from the oceanic surface.
- C.atmospheric refraction of starlight.
- D.magnetic field of Earth.
Correct answer
Explanation
The correct answer is C, atmospheric refraction of starlight. Starlight bends as it passes through layers of air of changing temperature and density, and because these layers keep moving, the star's apparent position and brightness flicker. A star is so far away that it behaves almost like a point source of light. As its light enters the Earth's atmosphere, it is refracted by air layers whose refractive index keeps changing. So the amount of light reaching our eyes rises and falls quickly, and the star seems to twinkle. Planets are much closer and appear as small discs; the flickering from different points of the disc averages out, so planets do not twinkle. The same refraction makes stars look slightly higher than they really are. Option A is wrong because twinkling has nothing to do with the colours or frequencies a star emits. Option B is wrong because starlight reaches us directly, not by reflection from the sea. Option D is wrong because the Earth's magnetic field does not bend visible light. Exam tip: stars twinkle (point source + atmospheric refraction); planets do not (extended source).
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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