CDS (II) 2024 General Knowledge · Question 21 of 120

Correct — C, Both 1 and 2. Both uses follow from one property and one number. X-rays penetrate matter, which makes the first use possible: a source is placed on one side of a welded joint and a film or detector on the other, and any crack, gas pocket or slag inclusion inside the metal lets more radiation through and shows up as a darker patch. This is industrial radiography, and reference accounts note that the great bulk of it is the testing and grading of welds on piping, pressure vessels, storage containers and pipelines — inspection that finds flaws without cutting the part open. The second use follows from wavelength. X-ray wavelengths are about the same size as the spacing between atoms in a crystal, so a crystal acts as a three-dimensional grating and scatters an X-ray beam into a pattern of spots whose positions and intensities can be worked back to the arrangement of the atoms. That is X-ray crystallography, the technique that gave chemistry the shapes of countless molecules and gave biology the double helix.

• (a)1 only — This accepts the flaw-detection use but rejects crystallography, which is if anything the more celebrated of the two. Determining crystal and molecular structure by X-ray diffraction has been standard practice for more than a century.
• (b)2 only — This accepts crystallography but rejects radiography of welds, which is routine industrial practice and a required inspection on pressure vessels and pipelines.
• (d)Neither 1 nor 2 — Both statements are true, so rejecting both is doubly wrong. The same two properties — penetration and atomic-scale wavelength — also underlie medical radiography and airport baggage screening.

X-rays are electromagnetic waves of much shorter wavelength and much higher energy than visible light, discovered by Roentgen in 1895 and produced when fast electrons strike a metal target. Being uncharged, they are not bent by electric or magnetic fields. Their short wavelength gives them two distinct powers: they pass through matter that light cannot, and they interact with the regularly spaced atoms of a crystal in a way that reveals where those atoms sit.

Statement items are best handled by asking, for each statement separately, what property would have to be true for it to work. Penetration explains everything on the imaging side, from a chest radiograph to a weld inspection to a scanned suitcase. Wavelength explains the analytical side, because a wave can only resolve detail on the scale of its own wavelength, and X-ray wavelengths happen to match the spacing of atoms. The most famous single result of the second use is worth remembering, since CDS has asked about it directly: the diffraction image known as Photo 51, taken at King's College London in 1952, was the X-ray evidence from which the double-helix structure of DNA was worked out.

• Industrial radiography places the specimen between an X-ray source and a film or detector, and internal flaws show as changes in film density.
• Most industrial radiography is the testing and grading of welds on piping, pressure vessels, storage containers and pipelines.
• X-ray wavelengths are comparable to the spacing between atoms in a crystal, which is why crystals diffract them into a pattern that reveals atomic positions.
• X-rays were discovered by Wilhelm Roentgen in 1895 and are produced when fast-moving electrons strike a metal target.
• Being uncharged, X-rays are not deflected by electric or magnetic fields, unlike beta or alpha radiation.

• Assuming X-rays are charged particles; they are electromagnetic waves and are not deflected by fields.
• Restricting X-rays to medicine; industry and crystallography use them just as heavily.
• Confusing X-rays with gamma rays — the wavelengths overlap, and the two are told apart by origin, not by a sharp boundary.

Usually as a properties-of-X-rays item or as a uses-based statement pair; the DNA connection and the position of X-rays in the electromagnetic spectrum are the recurring extensions.

• practice — not a real PYQ

  Internal cracks in a welded steel joint can be detected without cutting the joint open by using

  • (a)X-ray radiography
  • (b)a magnifying glass
  • (c)a litmus test
  • (d)an ordinary photograph

  Answer(a) X-ray radiography — the rays pass through the metal, and a flaw inside lets more radiation through, showing up on the film.

• practice — not a real PYQ

  X-rays can be used to determine the arrangement of atoms in a crystal chiefly because

  • (a)they are absorbed completely by crystals
  • (b)their wavelength is comparable to the spacing between atoms
  • (c)they carry an electric charge
  • (d)they travel faster than visible light

  Answer(b) their wavelength is comparable to the spacing between atoms — that is what lets the crystal act as a grating and produce a diffraction pattern.