NDA (I) 2022 General Ability Test · Question 78 of 150

Correct — C, refraction of starlight. As light from a distant star passes through the atmosphere it is bent (refracted) by layers of air whose density and temperature keep changing. Because those layers are constantly shifting, the star's apparent position and brightness flicker moment to moment — this atmospheric refraction is the twinkling (scintillation). Planets, being nearer and seen as small discs rather than points, twinkle far less.

• (a)diffraction of starlight — Diffraction is the bending of light around obstacles or edges; it is not what produces the flickering of stars.
• (b)reflection of starlight — Reflection is light bouncing off a surface — starlight reaching us is not bounced off the atmosphere, so this is not the cause.
• (d)dispersion of starlight — Dispersion splits light into its colours (as in a prism or rainbow); it can make a star briefly flash colours, but the twinkling itself is atmospheric refraction.

Atmospheric refraction is the bending of light as it travels through air layers of continuously varying refractive index (which depends on density and temperature). Light from a point source like a star is deflected slightly and by an ever-changing amount, so the star seems to shimmer — this is scintillation, or twinkling.

'Dispersion' is the tempting wrong choice because stars sometimes appear to change colour; but the actual position-and-brightness flicker comes from refraction through moving air layers, not from colour splitting.

• The atmosphere is made of layers of varying refractive index; light bends towards the optically denser layer.
• Continuous, changing refraction makes a star's apparent position and brightness fluctuate — twinkling.
• Stars are effectively point sources, so the effect is very noticeable; planets appear as tiny discs and twinkle much less.
• The same atmospheric refraction makes the Sun visible slightly before actual sunrise and after actual sunset.

• Choosing 'dispersion' because stars sometimes flash colours — the flicker itself is refraction.
• Mixing up refraction with reflection or diffraction, which do not cause twinkling.

A direct one-liner — 'twinkling of stars is due to' — with the other optics terms (reflection, diffraction, dispersion) as distractors.

No directly related past PYQ was found.

• practice — not a real PYQ

  Planets, unlike stars, generally do not twinkle because

  • (a)they emit their own light
  • (b)they are seen as extended discs, so the flickering from many points averages out
  • (c)they are outside the atmosphere
  • (d)they are much hotter than stars

  Answer(b) they are seen as extended discs, so the flickering from many points averages out.

• practice — not a real PYQ

  The Sun is visible a little before actual sunrise and after actual sunset because of

  • (a)reflection of light
  • (b)atmospheric refraction of light
  • (c)total internal reflection
  • (d)scattering of light

  Answer(b) atmospheric refraction of light — the same bending of light that makes stars twinkle.