In which region of the electromagnetic spectrum does the Lyman series of hydrogen lie?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → ultraviolet
The Lyman series consists of transitions ending in the ground state, n = 1. Its wavelengths lie between the first line and the series limit:
\( \displaystyle \lambda_{2\rightarrow1} = \frac{4}{3R} \approx 122\,\mathrm{nm}, \qquad \lambda_{\infty\rightarrow1} = \frac{1}{R} \approx 91\,\mathrm{nm} \)
All of these are shorter than about 400 nm, so the series lies in the ultraviolet.
Why the other options are incorrect
Option (1) visible — Incorrect. The visible lines of hydrogen form the Balmer series, ending in n = 2.
Option (2) infrared — Incorrect. The Paschen, Brackett and Pfund series, ending in n = 3, 4 and 5, lie in the infrared.
Option (4) x-ray — Incorrect. The largest energy change in hydrogen is 13.6 eV, far below the keV energies of X-rays.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), but it is still in the CUET 2026 syllabus, so do not skip it.
NCERT (Pre-rationalised edition, 2022-23):
Spectral Series – “… The Lyman series is in the ultraviolet, and the Paschen and Brackett series are in the infrared region.”