Physics
Atomic Spectra
2,777 words, 10 sections. The first two are free.
The opening sections
Definitions first, then the reasoning. Each section is short enough to read in one sitting.
This chapter is Physics unit 15, Atomic Spectra, of the PMDC MDCAT-2025 uniform curriculum, inside Physics, which carries 36 of the 180 marks in the paper. It has been asked 172 times across 71 official papers between 2000 and 2026, and the question bank holds 914 active questions on it at a mean difficulty of 1.58. The weight is heavily concentrated: atomic spectra and the Bohr model carry 90 distinct questions, X-rays another 55 and lasers 27, so the model and the series built on it are where the marks sit. A student who has mastered the chapter can name a series from the level its electrons land on, place it in the right region of the spectrum, and turn a transition into a wavelength.
A spectrum is radiation spread out by wavelength. A hot dense body emits every wavelength, giving a continuous spectrum, while isolated atoms emit only certain wavelengths, giving a line spectrum. An emission spectrum shows bright lines on a dark background, produced as excited atoms drop to lower states, and an absorption spectrum shows dark lines on a continuous background at the wavelengths the atoms absorb. The spectrum obtained from the emission and absorption of radiation by the atoms of a gas is a line spectrum, and hydrogen is the gas an evacuated tube is filled with to study it. Electron transitions in atoms release ultraviolet, visible or infrared photons, but never gamma rays, which come from nuclear processes rather than atomic orbits.
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