物理学中，原子谱线是指原子内部电子跃迁形成的谱线，可分为两类：

这两类谱线中所对应的两个跃迁能级需要对应着电子的束缚态，因而这类跃迁有时也被称为“束缚态-束缚态”跃迁，与之对应的是电子从束缚态获取足够的能量从而从原子中完全逸出（“束缚态-自由态”跃迁）。自由态的电子具有连续谱，此时的原子被电离，而过程所辐射的能谱也是连续的。

跃迁中辐射或吸收的单个光子所携带的能量等于电子跃迁的两个能级之差，用普朗克关系式即描述为能级差，其中是光子的频率，而是普朗克常数。

Spectroscopy /spɛkˈtrɒskəpi/ is the study of the interaction between matter and electromagnetic radiation. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, by a prism. Later the concept was expanded greatly to include any interaction with radiative energy as a function of its wavelength or frequency. Spectroscopic data is often represented by a spectrum, a plot of the response of interest as a function of wavelength or frequency.