CDS (I) 2022 General Knowledge · Question 22 of 120

Correct — B, atmospheric refraction of starlight. A star is so far away that it reaches us as a point source, and the light from that point has to cross an atmosphere made of layers at different temperatures and densities, which therefore have different optical densities. Light bends on passing from one such layer into the next, and because the air is never still, the amount and direction of that bending change from moment to moment. The apparent position of the star wanders slightly and the amount of light reaching the eye rises and falls — that flickering is the twinkling. Two consequences confirm the mechanism. Planets, which are near enough to present a small disc rather than a point, hardly twinkle at all, because the fluctuations from different parts of the disc average out; and stars viewed from above the atmosphere do not twinkle at all, which is one of the reasons for putting telescopes in orbit.

• (a)atmospheric reflection of starlight. — Reflection sends light back from a surface. There is no surface in the sky doing that to starlight, and reflection would not produce a rapid flicker in the star's brightness even if there were.
• (c)continuous change in the position of the star. — Stars do move, but the changes are measurable only over years and centuries, not over the second or two in which a star is seen to twinkle. If the star itself were the cause, the twinkling would not stop when the observer leaves the atmosphere — and it does.
• (d)oscillation of starlight. — Not a physical process. Starlight does not pulse in brightness on its own; what changes is the path it takes through the air between the star and the observer.

Refraction is the bending of light when it passes between media of different optical density. The atmosphere is not one medium but a stack of them, continuously stirred, so starlight is refracted many times over on the way down and the refraction keeps changing. The same effect explains several other everyday sights: the Sun is visible for a couple of minutes before it has actually risen and after it has actually set, because refraction lifts its apparent position; the Sun and Moon look flattened near the horizon; and the air above hot ground shimmers. Stars near the horizon twinkle most, because their light travels through the greatest thickness of atmosphere.

A useful test for questions of this kind is to ask what would happen if the atmosphere were removed. Twinkling stops, which places the cause in the air rather than in the star and eliminates two of the four options at once. Between reflection and refraction, refraction is the process that operates when light passes THROUGH a medium of changing density, which is what starlight does. Astronomers treat the effect as a nuisance rather than a curiosity — it is what limits the sharpness of ground-based images, and modern large telescopes use adaptive optics, deforming a mirror hundreds of times a second, to undo the distortion as it happens.

• Twinkling, or scintillation, is caused by atmospheric refraction of starlight through air layers of changing density.
• Stars behave as point sources, which is why the fluctuation is visible; planets present a small disc and the fluctuations average out.
• Stars close to the horizon twinkle more, because their light passes through a greater thickness of atmosphere.
• Above the atmosphere stars do not twinkle, which is one reason telescopes are placed in orbit.
• The same refraction makes the Sun visible slightly before actual sunrise and after actual sunset.

• Reaching for scattering, which explains the colour of the sky and of the setting Sun but not the flicker of a star.
• Choosing reflection because both words describe light changing direction.
• Assuming the star itself varies in brightness, which would not explain why the effect vanishes above the atmosphere.

As a straight cause question, as here, or bundled into a statements item listing several atmospheric optical effects to be sorted.

• practice — not a real PYQ

  Planets do not twinkle in the way stars do, mainly because

  • (a)they shine by reflected sunlight
  • (b)they are much closer and present a disc rather than a point source
  • (c)they lie outside the Earth's atmosphere
  • (d)their light is of a longer wavelength

  Answer(b) they are much closer and present a disc rather than a point source — the fluctuations from different parts of the disc average out, so the light arriving is steady.

• practice — not a real PYQ

  The Sun is visible for a short while before it has actually risen above the horizon. This is because of

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

  Answer(c) atmospheric refraction — light from the Sun is bent as it passes through air of increasing density, so the Sun appears higher than it really is.