Read the following passage and answer the questions.
A glass discharge tube is filled with hydrogen gas at low pressure. A high voltage is applied across the tube, and an electric current passes through the gas. The tube then glows. A spectrometer is used to study the light that comes out of the tube. In Bohr’s model, the electron of a hydrogen atom can stay only in certain allowed energy states, called stationary states. The energy of the electron in the \(n\)th state is \(E_n=-13.6/n^{2}\ \mathrm{eV}\), where \(n = 1, 2, 3, \ldots\) In the tube, fast-moving electrons collide with the hydrogen atoms. These collisions raise some atoms to states of higher energy. Such an excited atom does not stay in that state for long. It soon falls back to a lower state and gives out a photon.
The high voltage is now switched off, and white light is passed through the tube. The light that comes out is studied with the spectrometer. What does the spectrometer show?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → dark lines at wavelengths found in the emission line spectrum
An atom absorbs a photon only when the photon energy matches the gap between two of its energy levels. Emission lines come from the same gaps.
Option (1) bright lines on a dark background — Incorrect. Bright lines on a dark background form the emission line spectrum. Here the gas takes energy out of the white light and does not add any.
Option (2) a continuous spectrum with no lines — Incorrect. The atoms absorb photons of certain frequencies. So dark lines appear in the continuous spectrum of the white light.
Option (3) dark lines at wavelengths found in the emission line spectrum — Correct. The photons absorbed have energies equal to the gaps between levels. These are the energies of the photons given out in emission, so the dark lines fall at the emission wavelengths.
Option (4) dark lines only at wavelengths not present in the emission line spectrum — Incorrect. Each absorbed photon must match a gap between two levels. The atom gives out a photon of the same energy when it falls back across that gap. So every dark line lies at an emission wavelength.
NCERT (2026-27):
Atomic Spectra – “… When white light passes through a gas and we analyse the transmitted light using a spectrometer we find some dark lines in the spectrum. These dark lines correspond precisely to those wavelengths which were found in the emission line spectrum of the gas. …”