Twinkling of stars is primarily due to the atmospheric
- (a)refraction
- (b)reflection
- (c)polarization
- (d)dispersion
Correct — A, refraction. Starlight passes through many layers of the atmosphere that differ in temperature and density, and these layers keep changing. As the light refracts (bends) through this constantly shifting air, the apparent position and brightness of the star flicker from moment to moment — that is the twinkling. Because the effect comes from the bending of light by unsteady air, it is atmospheric refraction.
- (b)reflection — Reflection would simply bounce light off a surface; twinkling is produced by light bending as it travels through changing layers of air, not by reflection.
- (c)polarization — Polarization concerns the orientation of light waves and does not cause the flickering brightness of stars.
- (d)dispersion — Dispersion is the splitting of white light into its colours; while stars can look slightly coloured, the flickering (twinkling) itself is caused by refraction, not dispersion.
The atmosphere is made of layers with different densities, and hence different refractive indices, that are continually moving and mixing. Light from a distant point-like star is bent by varying amounts as these layers shift, so the star's image dances slightly and its brightness rises and falls — twinkling, technically called stellar scintillation. Planets, being nearer and seen as small discs rather than points, twinkle far less.
Twinkling needs an atmosphere and it involves the bending of light, so the answer is refraction. Astronauts above the atmosphere see stars shine steadily, which confirms the effect is atmospheric.
- Twinkling of stars (stellar scintillation) is caused by atmospheric refraction of starlight.
- The atmosphere's layers vary in density and refractive index and are constantly moving, so the bending keeps changing.
- Planets twinkle much less because they appear as small discs, not point sources.
- Above the atmosphere (for example, from space) stars do not twinkle.
- Choosing reflection or dispersion — twinkling is the bending (refraction) of light by moving air.
- Thinking planets twinkle like stars — planets are extended discs and twinkle far less.
Asked as the cause of twinkling — link the flicker to the atmospheric refraction of starlight.
Consider the following statements: A person in a spaceship located half-way between the earth and the sun will notice that the I. Sky is jet black. II. Stars do not twinkle. III. Temperature outside the spaceship is much higher than that on the surface of the earth. Of these statements
- (a) Only III is correct
- (b) I and II are correct
- (c) I and III are correct
- (d) I, II and III are correct
Answer(b) I and II are correct
Same concept from the other side — half-way to the Sun there is no atmosphere, so stars do not twinkle. Twinkling is an atmospheric-refraction effect, exactly what this NDA item states.
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.
Answer(c) atmospheric refraction of starlight.
The very same fact — that 2021 (I) NDA item also answers that the twinkling of stars is due to atmospheric refraction of starlight.
- practice — not a real PYQ
Planets do not twinkle like stars because they
- (a)are much hotter
- (b)appear as extended discs rather than point sources
- (c)emit their own light
- (d)lie outside the atmosphere
Answer(b) appear as extended discs — averaging over the disc cancels most of the flicker.
- practice — not a real PYQ
The Sun is visible about two minutes before actual sunrise mainly because of
- (a)atmospheric reflection
- (b)atmospheric refraction
- (c)dispersion
- (d)scattering
Answer(b) atmospheric refraction — light bends as it passes through the atmosphere.