CDS (II) 2024 General Knowledge · Question 23 of 120

Correct — D, both refraction and internal reflection of sunlight. Follow a single ray of sunlight into a raindrop. It bends as it enters the water, because it is passing from air into a denser medium — that is refraction, and because the different colours bend by different amounts, this is also the step that splits white light into its colours. The ray then strikes the far surface of the drop from the inside at a steep angle and is reflected back — internal reflection. Finally it bends again on leaving the drop, which widens the separation between the colours still further. So the light is refracted twice and reflected once inside the drop, and the description that covers the whole event is the one naming both processes together. The options that name only reflections are incomplete, since reflection alone cannot separate colours; without refraction there would be a bright arc but no spectrum in it.

• (a)single refraction of sunlight — Refraction is genuinely involved, but it happens twice, once on entering the drop and once on leaving, and the account also leaves out the internal reflection that turns the light back towards the observer.
• (b)one internal reflection of sunlight — A primary rainbow does involve exactly one internal reflection, so this is a true statement as far as it goes — but it is only part of the story. Reflection alone cannot split white light into colours; the refractions are what produce the spectrum.
• (c)two internal reflections of sunlight — Two internal reflections describe the secondary rainbow, the fainter outer bow with its colour order reversed. The ordinary primary bow has one, so this option names the wrong count and still omits refraction.

A rainbow is produced by sunlight interacting with spherical raindrops. Three effects work together: refraction, the bending of light as it crosses between air and water; dispersion, the fact that different colours bend by different amounts so that white light spreads into a spectrum; and internal reflection at the back surface of the drop, which sends the light back towards the observer. The result is a circular arc seen with the Sun behind the viewer, always centred on the point opposite the Sun.

The item can be answered by elimination, since three options each describe one piece of the process and only the fourth is a complete statement. But the geometry repays learning because it explains everything anyone notices about a rainbow. Red emerges from a drop at about forty-two degrees from the incoming sunlight and violet at about forty, which is why the primary bow shows red on the outside and violet on the inside, and why the bow always appears at the same angular size. A second, fainter bow sometimes appears outside the first; it comes from light that bounced twice inside the drops, which costs it intensity and reverses its colour order. And a rainbow has no fixed position on the ground — it depends on where the observer stands, so no two people ever see quite the same one.

• In a primary rainbow the light is refracted on entering a drop, internally reflected once, and refracted again on leaving.
• Dispersion during refraction is what separates white light into colours; reflection alone cannot do it.
• The primary bow shows red on the outside and violet on the inside, at roughly 42 and 40 degrees respectively.
• The secondary bow arises from two internal reflections, is fainter, and has its colours reversed.
• A rainbow is always seen with the Sun behind the observer, centred on the point directly opposite the Sun.

• Choosing the option that names one internal reflection because it is true of a primary bow; the item asks what the phenomenon involves, and refraction cannot be left out.
• Attributing the colours to reflection; it is refraction with dispersion that separates them.
• Confusing the primary and secondary bows — one internal reflection against two, and the colour order reversed.

Usually as a which-phenomena-are-involved item, or by asking for the number of internal reflections in the primary bow; the prism and dispersion questions sit alongside.

• practice — not a real PYQ

  In the secondary rainbow, the number of internal reflections undergone by light inside a raindrop is

  • (a)one
  • (b)two
  • (c)three
  • (d)none

  Answer(b) two — the extra reflection makes the secondary bow fainter and reverses its colour order compared with the primary bow.

• practice — not a real PYQ

  The separation of white sunlight into its component colours inside a raindrop is chiefly due to

  • (a)reflection
  • (b)dispersion during refraction
  • (c)absorption by water
  • (d)diffraction at the drop's edge

  Answer(b) dispersion during refraction — different colours bend by different amounts as they cross into and out of the water, which spreads them into a spectrum.