Twinkling of a star is due to :

Correct — B, Refraction of light. The atmosphere is not one uniform medium but a stack of layers whose density, and so whose refractive index, changes with height — and, crucially, keeps changing from moment to moment as warm and cool air move about. Starlight entering it is refracted continuously on its way down, bending towards the normal, so a star near the horizon appears a little higher than it really is. Because the air is restless, that bending is not steady: the apparent position of the star shifts slightly all the time, and the amount of starlight reaching the eye rises and falls with it. The star seems now brighter, now fainter, and that flicker is what we call twinkling. The reason stars twinkle and planets do not follows from the same argument. A star is so far away that it is effectively a point source, so the whole of its light wanders together. A planet is close enough to be an extended source — a small disc rather than a point — and can be treated as a great many point sources side by side. Their fluctuations are independent, so the brightening of one part is cancelled by the dimming of another, the total averages out, and the planet shines steadily.

• (a)Interference of light — Interference is the superposition of coherent waves producing bright and dark bands, as in Young's double-slit experiment or the colours of a soap film. Starlight reaching us is not coherent in the way that effect needs, and twinkling has no fringe pattern.
• (c)Polarization of light — Polarisation restricts the vibrations of light to one plane. It explains why the blue of the sky is partly polarised and why sunglasses can cut glare, but it changes no star's apparent brightness from second to second.
• (d)Diffraction of light — Diffraction is the bending of light round obstacles and through small apertures. It sets the limit on what a telescope can resolve and produces the coloured corona sometimes seen around the Moon, but the flicker of a star comes from refraction in air of continually changing refractive index.

Atmospheric refraction is the bending of light as it passes through air whose refractive index changes with height, and it accounts for a cluster of everyday sights that examiners like. The Sun is visible about two minutes before it actually rises and for about two minutes after it has actually set, because its light is bent over the horizon. The Sun's disc looks flattened at sunrise and sunset for the same reason. Objects seen over a hot road or a fire appear to waver, because the air just above the hot surface is less dense and constantly in motion. The twinkling of stars is that same local shimmer, seen on the scale of the whole atmosphere.

Four optical phenomena are offered and only one of them involves a change of medium, which is the quickest way in. Refraction happens when light crosses from one optical density to another; interference, diffraction and polarisation are all effects light shows without needing any such crossing. Since the twinkling stops entirely when you go above the atmosphere — this is one of the reasons for putting telescopes in orbit — the atmosphere must be doing it, and the atmosphere's optical role is refraction. Two extensions are worth carrying. First, planets do not twinkle, and knowing why is a favourite follow-up question. Second, the same effect blurs images from ground-based telescopes, which is why large observatories are built on high, dry mountains where there is less air overhead and the air that remains is steadier.

• The twinkling of stars is caused by atmospheric refraction of starlight, in a medium whose refractive index changes gradually with height.
• Because the physical conditions of the atmosphere are never stationary, the star's apparent position and brightness fluctuate.
• Stars are so distant that they behave as point sources; planets are extended sources whose many points average out the fluctuation, so planets do not twinkle.
• A star near the horizon appears slightly higher than its true position because the atmosphere bends starlight towards the normal.
• The same refraction makes the Sun visible about two minutes before actual sunrise and about two minutes after actual sunset, and flattens its disc at the horizon.

• Choosing diffraction because twinkling is a fluctuation; diffraction produces a fixed pattern, not a flicker.
• Attributing twinkling to something happening at the star, such as a change in its own brightness; the star is steady and the air is not.
• Forgetting the planets. A question that asks why planets do NOT twinkle is answered by their being extended sources, not by anything about their distance from the Sun.

Almost always as a one-line cause question like this, and its twin — 'why do planets not twinkle' — appears just as often, sometimes together in a statements format.

• practice — not a real PYQ

  Planets do not appear to twinkle mainly because:

  • (a)they shine by reflected light
  • (b)they are extended sources, so the fluctuations average out
  • (c)they lie inside the Earth's atmosphere
  • (d)their light is polarised

  Answer(b) they are extended sources, so the fluctuations average out — a planet acts as a great many point sources whose brightening and dimming cancel, unlike a star, which is effectively a single point.

• practice — not a real PYQ

  The Sun is seen a couple of minutes before it has actually risen above the horizon. This is due to:

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

  Answer(a) atmospheric refraction — the atmosphere bends the Sun's light over the horizon, giving an advance sunrise and a delayed sunset of about two minutes each.