Emission and Absorption Line Spectra - A Level Physics

From earth, we can figure out the chemical composition of a star looking at the absorption spectrum it produces. But why do we see absorption spectra from stars? A star is a source of light. Studying its light waves through a spectrometer or a prism, we will find a continuous spectrum. A continuous spectrum is made up of a continuous range of wavelengths.

A star like our sun emits photons of all frequencies to produce a continuous spectrum. But before these photons reach us on Earth, some photons get absorbed by the cooler gas around the star. The gas can then re-emit photons, giving us the emission spectrum if we can isolate just the gas and look at it through a spectroscope. But looking at the sun light that has passed through the gas, then we will get the absorption spectrum.

In other words, the photons that form the absorption spectrum are the ones that did not get absorbed by the gas.











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