# Relativistic mass

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Relativistic mass is a mass, e.g.  of a particle (or an object) that any observer is seeing in their particular reference system of coordinates. It is the same as invariant mass (usually denoted in physics by ) of particle (or object), multiplied by  factor, which tells how much mass (or rather enery) the moving particle has in relation to invariant mass e.g. . The invariant mass is sometimes called rest mass and then denoted by . E.g. .
The "mass" on the right side of Einstein's equation  is invariant (or rest mass) for a physicist and generally relativistic mass, or "inertial mass", for non physicists, which is rather non intuitive arrangement but luckily the Einstein's equation works for both kinds of mass (unless one wants specially accurate terminology, which then may involve also curvatures of spacetime).
Energy of each photon , where  is Planck constant,  is frequency of light,  is speed of light in vacuum, and  is the wavelength of light, carried by this photon, to which our eyes react with seeing each photon frequency (or wavelengh) as a separate color. We see only light wavelengths between about 400 (violet end of spectrum) to 800 (red end of spectrum) nanometers (one nanometer being equal ), while other animals may have different eye sensitivities. Knowing the wave length (or color of the photon) we may calculate the relativistic mass of this photon. And if the light is very dimm and we were cats (humans don't have eyes sensitive enough and they need hundreds of photons to see anything) we could even see single photons as flashes of light on the retina of our eyes.