The wavelength of X-rays is of the order of

The correct answer is A, 1 Å. X-rays have wavelengths of roughly 0.01 to 10 nanometres, and a typical value is about one ångström (1 Å = 10⁻¹⁰ m = 0.1 nm). This is about the size of an atom and the spacing between atoms in a crystal, which is why X-rays are diffracted by crystals and are used to study crystal structure. Their short wavelength means high energy, so they pass through soft tissue but are stopped by bone and metal, the basis of X-ray imaging. In the electromagnetic spectrum, in order of increasing wavelength, come gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves. Option B is wrong because one micrometre lies in the near-infrared region. Option C is wrong because one millimetre is at the boundary of microwaves and far infrared. Option D is wrong because one centimetre is in the microwave region, used in radar and microwave ovens. Exam tip: X-rays are about 1 Å; visible light is 400 to 700 nm.