Physics · Atoms · NEET
Bright lines on a dark background. When an atomic gas is excited (for example by passing electric current through it at low pressure), its electrons jump to higher levels and then fall back, giving out photons of certain fixed wavelengths only. These appear as bright coloured lines. The rest of the screen stays dark because no other wavelengths are given out. This is the emission line spectrum.
Because the energy gaps between levels are fixed for a given atom. In emission, an electron drops from a higher to a lower level and gives out a photon of energy E = E(high) - E(low). In absorption, an electron uses a photon of that same energy to jump from the lower to the higher level. Same energy gap means same frequency and same wavelength, so the bright emission line and the dark absorption line sit at the identical place. That is why the spectrum acts as a fingerprint of the element.
In emission you look only at light the gas itself sends out, so away from the special lines there is nothing to see, hence a dark background with bright lines. In absorption you shine white light (a continuous spectrum) through a cooler gas. The gas removes only its own special wavelengths, leaving dark gaps in an otherwise full bright spectrum, hence a bright background with dark lines.
NCERT gives h(nu_if) = E(n_i) - E(n_f). When an electron jumps from a higher state n_i to a lower state n_f, a photon of frequency nu_if is emitted (emission line). When an atom absorbs a photon of exactly that frequency, the electron goes from the lower to the higher state (absorption line). The same equation controls both, so the frequencies match.
They are absorption lines. The hot dense core of the Sun gives a continuous spectrum. Cooler gases in the Sun's outer atmosphere absorb their characteristic wavelengths as this light passes out, leaving dark lines. This tells us which elements are present in the Sun, showing how absorption spectra identify materials from far away.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT states that when an atomic gas or vapour is excited at low pressure, usually by passing an electric current through it, the emitted radiation has a spectrum containing certain specific wavelengths only. This is called the emission line spectrum and it consists of bright lines on a dark background.
When white light passes through a gas and the transmitted light is analysed with a spectrometer, some dark lines appear. These dark lines correspond precisely to the wavelengths found in the emission line spectrum of that gas. This is the absorption spectrum of the gas.
Each element emits and absorbs its own fixed set of wavelengths, decided by its own energy levels. No two elements have the same set, so the line pattern uniquely identifies the element, just like a fingerprint identifies a person.
Hydrogen shows both. Excited hydrogen gives an emission line spectrum with bright lines (the Balmer, Lyman and other series). The same hydrogen can also produce an absorption spectrum with dark lines at those same wavelengths when white light passes through cool hydrogen gas.
The lines are produced by electron transitions between discrete energy levels. NCERT: spectral lines appear when electrons jump from a higher energy state to a lower state and photons are emitted (emission lines), or when an atom absorbs a photon of exactly the right energy to jump up (absorption lines).
