NDA (II) 2022 General Ability Test · Question 65 of 150

Correct — D, 1 nm. X-rays occupy the band from roughly 0·01 nm to about 10 nm — that is 0·1 Å to 100 Å — lying between ultraviolet on the long side and gamma rays on the short side. Of the four figures offered only 1 nm falls inside that band, and it is squarely the region used in medical radiography.

• (a)500 nm — 500 nm is visible light — green, near the middle of the roughly 380–750 nm visible band. It is about five hundred times too long to be an X-ray.
• (b)5000 nm — 5000 nm is 5 micrometres, well into the infrared — the heat-radiation region, and the longest wavelength on the list, so the furthest possible from an X-ray.
• (c)100 nm — 100 nm sits in the extreme ultraviolet, just beyond violet light. It is the nearest wrong answer, but still about a hundred times longer than a typical X-ray wavelength.

The electromagnetic spectrum is a single continuum ordered by wavelength — radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays, from longest to shortest. All of them travel at the speed of light in vacuum, so c = fλ ties wavelength and frequency inversely, and E = hf makes the shortest wavelengths the most energetic. X-rays sit near the energetic end, with wavelengths of the order of the spacing between atoms in a crystal, which is why they are used for crystallography as well as radiography.

The examiner is testing whether you carry rough numerical anchors for the spectrum, not a formula. Fix two of them and the question solves itself — visible light runs about 380–750 nm, and X-rays are three orders of magnitude shorter, around 0·01–10 nm. Every distractor here is longer than visible light's short end or inside the visible band, so only 1 nm survives. One honest nuance is worth carrying. X-rays and gamma rays overlap in wavelength and are separated by where they originate, with electron transitions and electron braking producing X-rays and the atomic nucleus producing gamma rays, rather than by any sharp cut-off — though exam keys use the textbook ladder.

• X-rays span roughly 0·01 nm to 10 nm, shorter than ultraviolet and longer than gamma rays.
• Visible light runs from about 380 nm (violet) to about 750 nm (red), so 500 nm is green light.
• Because c = fλ, the shorter the wavelength the higher the frequency and the more energetic the photon.
• X-rays are produced by firing high-energy electrons at a metal target, and are absorbed strongly by heavy elements — bone, and the barium swallowed before a stomach X-ray.
• X-ray and gamma-ray wavelengths overlap; the two are distinguished by where they come from, not by a fixed boundary.

• Mixing up nanometres and micrometres — 5000 nm is infrared, not an X-ray.
• Assuming ultraviolet and X-ray are neighbours of similar size; they differ by about two orders of magnitude.
• Thinking a shorter wavelength means a lower frequency.

Either as a pick-the-band question like this one, or as an ordering question — arrange gamma, X-ray, ultraviolet, visible and radio by increasing wavelength or decreasing energy.

• practice — not a real PYQ

  Which one of the following radiations has the longest wavelength?

  • (a)X-rays
  • (b)Ultraviolet
  • (c)Infrared
  • (d)Gamma rays

  Answer(c) Infrared — the spectrum runs gamma < X-ray < ultraviolet < visible < infrared in increasing wavelength.

• practice — not a real PYQ

  A radiation of wavelength 600 nm belongs to which part of the electromagnetic spectrum?

  • (a)X-rays
  • (b)Visible light
  • (c)Microwaves
  • (d)Gamma rays

  Answer(b) Visible light — the visible band is roughly 380–750 nm, and 600 nm is orange-red.