This open educational resource explains how atomic spectra arise from quantised electron energy levels and how atoms emit or absorb photons of specific energies. It introduces emission spectra, absorption spectra, hydrogen spectral series, electron transitions, photon energy, wavelength, and the relationship between spectral lines and atomic structure. The article is designed for students preparing for university-level physics and modern physics. It connects visible spectral evidence with invisible quantum structure, showing how discrete spectral lines reveal allowed electron energy levels inside atoms. The resource includes conceptual explanations, diagrams, worked examples, common misconceptions, exam strategies, practice questions, an extended problem bank, a glossary, external references, and links to related Prep4Uni.online learning pages. Main topics covered include atomic spectra, quantised energy levels, emission and absorption spectra, the hydrogen spectrum, Balmer and Lyman series, the Rydberg formula, Bohr’s interpretation of hydrogen spectra, photon energy calculations, spectral fingerprints, spectroscopy applications, and the use of atomic spectra in astronomy, laboratory analysis, environmental monitoring, materials science, plasma physics, and modern technologies. This PDF is an archived educational version of the Prep4Uni.online learning page intended for citation, sharing, and long-term preservation as an open educational resource.