The wavelength of a photon is found to be \( 5\times10^{-10}\,\mathrm{m} \). What do you expect the photon to be?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → X-rays
The band of the electromagnetic spectrum is fixed by the wavelength. In nanometres the given wavelength is:
\( \lambda=5\times10^{-10}\,\mathrm{m}=0.5\,\mathrm{nm} \)
Option (1) Gamma rays — Incorrect. Gamma rays have wavelengths from about \( 10^{-10}\,\mathrm{m} \) down to less than \( 10^{-14}\,\mathrm{m} \). So 0.5 nm is longer than this whole range.
Option (2) X-rays — Correct. The X-ray region covers wavelengths from about \( 10^{-8}\,\mathrm{m} \) (10 nm) down to \( 10^{-13}\,\mathrm{m} \), and \( 5\times10^{-10}\,\mathrm{m} \) lies well inside it.
Option (3) Infrared rays — Incorrect. Infrared waves run from 1 mm to 700 nm, more than a thousand times longer than 0.5 nm.
Option (4) Ultraviolet rays — Incorrect. Ultraviolet ends at about \( 6\times10^{-10}\,\mathrm{m} \) (0.6 nm) on its short-wavelength side, so 0.5 nm is already beyond the UV band.
NCERT (2026-27):
X-rays – “Beyond the UV region of the electromagnetic spectrum lies the X-ray region. We are familiar with X-rays because of its medical applications. It covers wavelengths from about 10–8 m (10 nm) down to 10–13 m (10–4 nm).”