Astronomers initially had difficulty to identify the emission lines in a quasar spectra at optical wavelengths because lines were 'red shifted'.
'Red shift':
It is a concept that can be understood as the wavelength of the light is stretched, so the spectral lines is seen as 'shifted' towards the red part of the spectrum.
A similar effect happens with sound waves when a source moves relative to an observer.
Therefore, astronomers initially had difficulty to identify the emission lines in a quasar spectra at optical wavelengths because lines were 'red shifted'.
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