The dark bands or lines observed on an emission spectrum are known as absorption lines. These lines indicate that certain wavelengths or frequencies of light are being absorbed by the medium through which the light passes.
When white light or light from a continuous spectrum source passes through a medium, such as a gas or a cooler outer layer of a star, the atoms or molecules in the medium can absorb specific wavelengths of light. These absorptions occur because the atoms or molecules have energy levels that are quantized, meaning they can only absorb or emit light at certain discrete wavelengths corresponding to the energy differences between their energy levels.
As a result, when the white light passes through the medium, the specific wavelengths of light that match the energy differences of the atoms or molecules in the medium are absorbed. This absorption appears as dark bands or lines on the spectrum, indicating the absence of those specific wavelengths of light.
Each element or compound has its own unique set of absorption lines, creating a distinct pattern or fingerprint on the spectrum. These absorption lines are used in spectroscopy to identify the elements or compounds present in a sample by comparing the observed absorption lines to known reference spectra.
It's worth noting that emission spectra, on the other hand, are produced when a medium emits light at specific wavelengths. These emission spectra appear as bright lines or bands on the spectrum, indicating the specific wavelengths of light that are emitted by excited atoms or molecules in the medium.