CDS (I) 2020 General Knowledge · Question 12 of 120

Correct — D, Ultraviolet (UV). Arrange the four by wavelength and the order is fixed: radio waves are the longest, running from millimetres to kilometres; infrared comes next, roughly 700 nanometres up to a millimetre; visible light occupies about 400 to 700 nanometres; ultraviolet lies just beyond the violet end at 100 to 400 nanometres. Ultraviolet is the only one of the four shorter than visible light, so it has the shortest wavelength and, correspondingly, the highest photon energy of the group.

• (a)Radio waves — The longest wavelengths on the list, and on the electromagnetic spectrum as a whole. This option is the exact opposite of what the question wants.
• (b)Visible light — Shorter than infrared and radio, but longer than ultraviolet. Visible light is the reference band the other three are described against, not the extreme.
• (c)Infrared (IR) — Infra means below — below the red end of the visible band in frequency, therefore longer in wavelength than visible light and much longer than ultraviolet.

The electromagnetic spectrum is one continuous family, ordered by wavelength: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Wavelength falls along that sequence while frequency and photon energy rise. Every member travels at the same speed in vacuum, so the only things that distinguish them are wavelength, frequency and energy — and the way each interacts with matter.

The prefixes carry the answer. Infra means below and ultra means beyond, both measured against the visible band, so infrared sits on the long side of red and ultraviolet on the short side of violet. Once that is fixed, the only remaining question is where radio waves go, and they are the longest of all. Nothing here needs a number. A related habit worth building is that the same ordering also ranks photon energy, in the opposite direction — which is why ultraviolet can burn skin and cause the body to make vitamin D while infrared merely warms it.

• Ultraviolet radiation spans about 100 to 400 nanometres, shorter than visible light and longer than X-rays.
• Visible light runs from roughly 400 nanometres at the violet end to 700 nanometres at the red end.
• Infrared lies beyond red, from about 700 nanometres up to the millimetre range.
• Radio waves have the longest wavelengths of the whole spectrum, from millimetres upwards.
• Ultraviolet makes up about 10 per cent of the Sun's total electromagnetic output; the ozone layer absorbs most of the wavelengths below 300 nanometres.

• Reading infrared as the short-wavelength member because it sounds energetic.
• Mixing up the direction of the wavelength and frequency scales; as one rises the other falls.
• Assuming visible light must be at one extreme because it is the band we see.

As shortest or longest wavelength among four named bands, or as a ranking of bands by energy per photon.

• practice — not a real PYQ

  Which one of the following types of electromagnetic radiation has the highest energy per photon?

  • (a)Microwaves
  • (b)Infrared
  • (c)Visible light
  • (d)Ultraviolet

  Answer(d) Ultraviolet — of these four it has the shortest wavelength, and photon energy rises as wavelength falls.

• practice — not a real PYQ

  The ozone layer in the stratosphere protects life on Earth chiefly by absorbing

  • (a)radio waves
  • (b)infrared radiation
  • (c)ultraviolet radiation
  • (d)microwaves

  Answer(c) ultraviolet radiation — ozone and molecular oxygen absorb most solar ultraviolet below about 300 nanometres.