Lesson 5 of 12 · 6 min
Atomic spectra
NCERT §12.3
Zoya shines a white torch through a flask of hydrogen and looks at it with the spectroscope. The rainbow now has thin dark gaps. Where have they seen those positions before?
The lesson in notes
In short
An atomic gas or vapour excited at low pressure, usually by passing a current through it, emits only certain specific wavelengths. This is an emission line spectrum: bright lines on a dark background.
Each element's line spectrum is its own, so it works like a fingerprint for identifying a gas.
Pass white light through the same gas and look at the transmitted light with a spectrometer: dark lines appear in the otherwise continuous spectrum. This is the absorption spectrum of the gas.
The dark absorption lines fall at exactly the wavelengths of the bright emission lines of that gas: an atom absorbs the same frequencies it emits.
Contrast: a hot solid gives a continuous spectrum; a rarefied gas of separate atoms gives lines.