Emitted light
Observed light

Redshift Calculator


The redshift calculator will help you to estimate the redshift parameter z. The discovery of the redshift in astrophysics has become a crucial point in understanding the history of our universe. In the further text, we explain what redshift is and how you can estimate its magnitude using the mathematical redshift definition. Finally, we are also discussing differences between redshift and blueshift.

What is redshift?

Redshift is a phenomenon where the spectral lines of electromagnetic radiation coming from some stars or galaxies are shifted towards longer wavelengths (or lower frequencies). Term redshift was introduced since the visible light of the longest wavelength is red (approximately 700 nm). Several reasons can cause the redshift:

  • relativistic Doppler effect - occurs whenever a light source moves away from an observer. You can check out our Doppler effect calculator which is a classical version of Doppler effect.
  • expansion of the universe - the more distant galaxies are, the faster they seem to move away from us.
  • gravitational effect - light loses energy by "overcoming" gravity of the star, i.e., the light increases its wavelength.

Redshift definition

In astronomy, the redshift is characterized by a dimensionless quantity called z. Our redshift calculator can estimate z parameter in two ways:

z = (λo - λe) / λe or z = (fe - fo) / fo


  • z is the redshift parameter,
  • λe and fe are respectively the wavelength and the frequency of emitted light,
  • λo and fo are respectively the wavelength and the frequency of observed light.

The possibility to use both wavelength and frequency results from the Planck's equation. You can see from above redshift formula that the greater z, the bigger the difference between emitted and observed light.

Redshift and blueshift

When the observed wavelength is smaller than emitted, the z parameter is negative (z < 0) and the blueshift phenomenon occurs. In this case, the spectral lines are shifted towards shorter wavelengths (blue has the shortest wavelength, approximately 450 nm). There are different interpretations of the z parameter, for example:

  • when Doppler effect causes the shift, the object approaches (blueshift) or recedes (redshift) from the observer,
  • we would observe a blueshift if the universe would shrink and redshift if the universe would expand,
  • when a gravitational effect causes the shift, the light is emitted from a source with a weaker (blueshift) or a stronger (redshift) gravitational field than the gravitational field in which the observer resides.
Dominik Czernia, PhD student

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