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 — C, atmospheric refraction of starlight. Starlight travels down through the Earth's atmosphere, whose density — and therefore refractive index — keeps changing with height and with turbulence. This continuously refracts (bends) the light by slightly varying amounts, so the star's apparent position and brightness keep shifting, which is the flickering we see as twinkling.
- (a)particular frequencies of the starlight. — Twinkling is not caused by particular frequencies; light of all frequencies from a star is affected the same way by the changing atmosphere, so frequency is not the cause.
- (b)reflection of starlight from the oceanic surface. — Twinkling has nothing to do with the ocean — it is seen everywhere, including over land and from high mountains, so reflection from water cannot explain it.
- (d)magnetic field of Earth. — The Earth's magnetic field does not bend visible light, so it cannot make stars twinkle.
The atmosphere is not uniform — its density and refractive index vary with altitude and are constantly stirred by temperature-driven turbulence. As starlight passes through, it is refracted by amounts that change from moment to moment, so the light reaching the eye fluctuates in apparent direction and intensity. This flickering is the twinkling of stars.
The key word is 'refraction'. Students may be pulled towards scattering or reflection, but twinkling is specifically an atmospheric-refraction effect. Note that planets, being closer and seen as small discs rather than points, average out these fluctuations and so do not twinkle noticeably.
- Stars twinkle because of atmospheric refraction — the refractive index of air varies with density and turbulence.
- The starlight is continuously refracted by tiny, changing amounts, so its apparent position and brightness fluctuate.
- Planets generally do not twinkle because they appear as small discs, averaging out the fluctuations.
- The same atmospheric refraction makes the Sun visible a few minutes before actual sunrise and after actual sunset.
- Confusing twinkling (refraction) with the blue sky or red sunset (scattering).
- Assuming planets twinkle in the same way as stars.
Asked as the cause of the twinkling of stars, or grouped with other atmospheric-refraction effects such as the apparent early sunrise.
Consider the following phenomena: 1. Size of the sun at dusk 2. Colour of the sun at dawn 3. Moon being visible at dawn 4. Twinkle of stars in the sky 5. Polestar being visible in the sky. Which of the above are optical illusions?
- (a) 1, 2 and 3
- (b) 3, 4 and 5
- (c) 1, 2 and 4
- (d) 2, 3 and 5
Answer(c) 1, 2 and 4 — including the twinkle of stars, an atmospheric optical effect.
Same concept — the twinkling of stars is listed here as an atmospheric optical effect, exactly the phenomenon this question attributes to atmospheric refraction.
Which of the following statements with regard to the phenomenon of the primary rainbow formation by water droplets is/are correct? 1. It involves refraction and one internal reflection of sunlight. 2. It involves refraction of sunlight only. 3. It is formed as the inner bow. 4. It may involve more than one internal reflection as well as refraction of sunlight. Select the answer using the code given below:
- (a) 1 only
- (b) 1 and 3
- (c) 3 and 4
- (d) 2 and 3
Answer(b) 1 and 3
Related — atmospheric optics of sunlight passing through the air; both rely on refraction of light in the atmosphere, the cause of twinkling here.
- practice — not a real PYQ
Stars appear to twinkle but planets generally do not, because
- (a)planets are larger and appear as extended discs, averaging out the fluctuations
- (b)planets emit their own light
- (c)planets are hotter than stars
- (d)planets have no atmosphere
Answer(a) planets are larger and appear as extended discs, averaging out the fluctuations — so their light does not flicker noticeably.
- practice — not a real PYQ
The Sun is visible a few minutes before actual sunrise mainly because of
- (a)reflection of light
- (b)atmospheric refraction
- (c)dispersion of light
- (d)total internal reflection
Answer(b) atmospheric refraction — it bends the Sun's light so we see it before it is geometrically above the horizon.