CDS (II) 2020 General Knowledge · Question 104 of 120

Correct — C, Thunderstorm. A lightning flash and the thunder that follows it are produced by the same event at the same instant — the sudden heating of a channel of air by the discharge — so any delay between seeing and hearing must come from the journey, not from the source. Light covers the few kilometres from cloud to observer in a few millionths of a second, effectively instantaneously; sound, travelling at roughly 330 to 344 metres per second in air, needs about three seconds for every kilometre. That is why the flash arrives first and the rumble follows, and why counting the seconds between them and dividing by three gives a rough distance to the strike in kilometres. No other option on the list involves two signals from one event racing each other, which is what a comparison of speeds requires.

• (a)Twinkling of stars in night sky — Twinkling is atmospheric refraction. Starlight passes through air layers of continually shifting density, so the apparent position and brightness of the star wobble. No sound is involved, so nothing is being compared.
• (b)Lighting of a matchstick — A struck match gives out light and a faint scratch of sound at once, but it is held at arm's length. Over a distance of half a metre sound arrives in about one and a half thousandths of a second — far too short an interval to notice, so nothing is demonstrated.
• (d)Mirage — A mirage is total internal reflection combined with the continuous refraction of light through air layers heated to different temperatures above a hot road or desert. It is purely an optical effect with no sound component.

Light in air travels at very nearly its vacuum speed of about 3 × 10⁸ metres per second, while sound in air travels at about 344 metres per second at 20 degrees Celsius — roughly a million times slower. Sound is a mechanical wave and needs a material medium; light is an electromagnetic wave and does not. The speed of sound rises with temperature and is much higher in liquids and higher still in solids, whereas the speed of light falls in a denser medium.

The item asks for evidence, not for a fact, so the test to apply to each option is whether it puts a light signal and a sound signal from the same event into a race the observer can watch. Only the thunderstorm does. The other three are all optical phenomena with no acoustic partner — which is the pattern of the distractor set and worth recognising, because the twinkling of stars, the mirage and the apparent bending of a pencil in water recur across CDS and NDA papers as refraction items. The everyday rule of thumb that follows from the answer is worth carrying: about three seconds of delay for each kilometre of distance to the lightning.

• The speed of light in air is about 3 × 10⁸ m/s; the speed of sound in air is about 344 m/s at 20 degrees Celsius.
• Lightning and thunder are produced simultaneously; the gap between them is entirely a difference in travel time.
• Roughly three seconds of delay corresponds to one kilometre of distance to the lightning strike.
• Sound needs a material medium and cannot travel through vacuum; light can.
• The speed of sound increases with temperature and is greater in liquids and solids than in gases, while the speed of light decreases in an optically denser medium.

• Picking twinkling or mirage because they sound like light phenomena; the question wants a comparison of two speeds, not an optical effect.
• Treating the delay as evidence that thunder is produced after the flash — both start together.
• Confusing the speed of light in air with its speed in glass or water, where it is appreciably slower.

As this evidence item, as a direct comparison of the two speeds, or through a numerical thunderstorm problem asking how far away the lightning struck.

• practice — not a real PYQ

  A person hears the thunder 6 seconds after seeing the lightning. Taking the speed of sound in air as about 340 m/s, the lightning struck at a distance of roughly

  • (a)0·5 km
  • (b)1 km
  • (c)2 km
  • (d)6 km

  Answer(c) 2 km — distance is speed multiplied by time, 340 × 6 ≈ 2040 m. The travel time of the light itself is negligible.

• practice — not a real PYQ

  Sound cannot travel through

  • (a)a solid
  • (b)a liquid
  • (c)a gas
  • (d)vacuum

  Answer(d) vacuum — sound is a mechanical wave and needs particles of a medium to pass the disturbance along.