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Both calculators run independently — change the inputs on either side to compare results.

Relativistic Doppler Effect Calculator

Compute the observed frequency and wavelength when a light source moves toward or away from you at relativistic speeds. Used in astrophysics to analyze spectra of fast-moving stars, galaxies, and spacecraft.

Source Frequency (Hz)

Relative Velocity (c)

Motion Direction

Fill in the required fields to see your result.

Relativity

Relativistic Velocity Addition Calculator

Adds two velocities correctly under special relativity so the result never exceeds the speed of light. Essential when objects move at a significant fraction of c.

First Velocity (m/s)

Second Velocity (m/s)

Fill in the required fields to see your result.

Key differences

	Relativistic Doppler Effect Calculator	Relativistic Velocity Addition Calculator
Category	Relativity	Relativity
Inputs required	3	2
Result	Observed Frequency (Hz)	Combined Velocity (m/s)
What it does	Compute the observed frequency and wavelength when a light source moves toward or away from you at relativistic speeds. Used in astrophysics to analyze spectra of fast-moving stars, galaxies, and spacecraft.	Adds two velocities correctly under special relativity so the result never exceeds the speed of light. Essential when objects move at a significant fraction of c.