An X-ray can be deflected :

Correct — D, neither by electric field nor by magnetic field. X-rays are electromagnetic radiation, the same kind of thing as light and radio waves but with wavelengths around an ångström, roughly a ten-thousandth of the wavelength of visible light. Electromagnetic radiation carries energy and momentum but no electric charge, and the force that an electric or magnetic field can exert on something is F = q(E + v × B), which is identically zero when the charge q is zero. So a beam of X-rays passes through the gap of the strongest available magnet and between the plates of a charged capacitor without bending by so much as a hair. This is precisely how the different radiations were told apart historically. Alpha particles are helium nuclei and carry a positive charge, so a magnetic field bends them one way; beta particles are electrons, negative and far lighter, so they bend the other way and much more sharply; gamma rays and X-rays are uncharged and go straight on. The same test separates X-rays from cathode rays, which are streams of electrons and are steered by both fields — that steering is what aims the beam in an old television tube. X-rays can be bent, but only by mechanisms that do not need charge: they are diffracted by the regular planes of atoms in a crystal, which is the basis of X-ray crystallography and of how the structure of DNA was read.

• (a)by a magnetic field. — True of cathode rays and of beta particles, both of which are moving electrons, but not of X-rays. A magnetic force acts only on moving charge, and there is none in an electromagnetic wave.
• (b)by an electric field. — An electric field pushes charge. X-ray photons are neutral, so the field has nothing to act on, and the beam is undeflected however strong the field.
• (c)by both electric and magnetic field. — This describes a charged particle beam — electrons in a cathode ray tube or in an electron microscope, which are focused and steered by exactly this pair of fields. X-rays behave the opposite way and respond to neither.

X-rays were discovered by Wilhelm Conrad Röntgen in November 1895, work for which he became the first recipient of the Nobel Prize in Physics in 1901. They are produced when fast electrons are stopped abruptly by a heavy metal target, part of their energy appearing as radiation of very short wavelength. On the electromagnetic spectrum X-rays lie between ultraviolet and gamma rays, with wavelengths of the order of 1 ångström, which is comparable to the spacing between atoms in a solid — the reason crystals diffract them and the reason they penetrate soft tissue while dense bone absorbs them.

The item is testing one idea only: does the thing carry charge. Sort the radiations by charge and every question of this family answers itself. Alpha is a helium nucleus with charge plus two, beta is an electron with charge minus one, and both are deflected, in opposite directions and by different amounts because their masses differ so widely. Gamma rays, X-rays, visible light and radio waves are all electromagnetic and all uncharged, so all pass straight through. Cathode rays are the trap sitting next door in the same chapter — they were once called rays, but they are electrons and they are deflected by both fields. The distinction is worth holding because two of the wrong options here are simply correct statements about cathode rays.

• X-rays are electromagnetic radiation with wavelengths of about 1 ångström, shorter than ultraviolet and longer than gamma rays.
• The force from electric and magnetic fields is F = q(E + v × B), which is zero for anything uncharged, so X-rays are deflected by neither.
• Alpha particles are positively charged and beta particles negatively charged, so both are deflected; gamma and X-rays are not.
• Cathode rays are electrons and are deflected by both fields, which is how a cathode ray tube steers its beam.
• X-rays are discovered by Röntgen in 1895 and are diffracted by crystals, a charge-free mechanism used in X-ray crystallography.

• Confusing X-rays with cathode rays; the second are electrons and are deflected by both fields.
• Assuming that anything called a 'ray' must be a stream of particles that a magnet can steer.
• Concluding that X-rays cannot be bent at all — crystals diffract them, which is a wave effect and needs no charge.

As a deflection question like this, as 'which is NOT a property of X-rays', or as an ordering of the electromagnetic spectrum by wavelength.

• practice — not a real PYQ

  A beam of alpha particles, a beam of beta particles and a beam of X-rays all pass through a strong magnetic field. Which one of them is undeflected?

  • (a)The alpha beam
  • (b)The beta beam
  • (c)The X-ray beam
  • (d)All three are undeflected

  Answer(c) The X-ray beam — alpha particles carry a positive charge and beta particles a negative one, so both bend, while X-rays are uncharged electromagnetic radiation and go straight on.

• practice — not a real PYQ

  The wavelength of X-rays is of the order of:

  • (a)1 metre
  • (b)1 millimetre
  • (c)1 micrometre
  • (d)1 ångström

  Answer(d) 1 ångström — that is 10⁻¹⁰ m, comparable to the spacing of atoms in a crystal, which is why crystals diffract X-rays.