Spectral Line Width:
The lines are commonly symmetric around the λmax, the wavelength at that the intensity of emitted radiation is maximum. An absorbance is also the maximum at the similar wavelength. The line width is described as the width of the signal measured at one half the height of the maximum signal. That is expressed as ?λ ½ and is measured in the units of wavelength. That full width is measured at one half of the maximum (FWHM) is also known as effective line width. Below figure shows the schematic diagram describing the atomic line width. Here the line width resulting from a transition of an electron among two discrete, single valued energy states is expected to be zero.
Figure: A schematic diagram defining the atomic line width
You might be wondering in which if the transitions leading to the atomic spectrum are amongst the discrete energy levels, then how do we get width or broadness within the signal.