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Is there anything like cold or warm light? Everything in this world has a temperature, What about light?

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  • $\begingroup$ Try thinking of temperature as a measure of the random kinetic energy of particles. What happens then? $\endgroup$ – user191954 Oct 3 '18 at 14:56
  • $\begingroup$ Light has speed. If we talk about kinetic energy, we need the mass of light too. Again what's the mass of light? $\endgroup$ – Krishna Deshmukh Oct 3 '18 at 15:08
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    $\begingroup$ Have you checked this well written answer: quora.com/Does-light-have-temperature .Also: physics.stackexchange.com/questions/59456/… $\endgroup$ – user190081 Oct 3 '18 at 15:23
  • $\begingroup$ Temperature is the mean kinetical energy of the particles. For visible light, it is some thousands C. The mass of the light can be calculated from $E=mc^2$, and typically you know the energy (for everyday sizes, it is very small - the largest atomic bomb radiated some kg of light away). $\endgroup$ – peterh Oct 3 '18 at 15:47
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    $\begingroup$ @peterh $mc^2$, where $m$ is the invariant mass, is not kinetic energy — it's rest energy. And for light it's zero. $\endgroup$ – Ruslan Oct 3 '18 at 16:06
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Blackbody radiation is defined as radiation that is in thermal equilibrium. Therefore it has a temperature. Other types of radiation do not have a well-defined temperature, by definition.

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When an object gets really hot it starts to give off visible and uv light, and an increased amount of IR. Color scientists used the term colour temperature to compare the colour of light to a known black body standard (you can google D65). A lot of cheaper led lights are 5000K which is blueish, the more expensive and slightly less efficient led bulbs are 3000K and are reddish closer to natural light. The 3000 Kelvin is the about the same color as a blackbody at 3000K.

But light does not have temperature like matter does which is mass and kinetic energy.

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    $\begingroup$ But light does not have temperature like matter does which is mass and kinetic energy. Not true. Blackbody radiation does have a temperature. For example, the cosmic microwave background has a temperature of 2.7 K. $\endgroup$ – Ben Crowell Oct 3 '18 at 21:08
  • $\begingroup$ Ben, according to wikipedia "The CMB has a thermal black body spectrum at a temperature of 2.72548±0.00057 K." So it is a spectrum not a temperature, if you stick your finger on it (if you could), your finger would get to 2.7 but radiation only has a color temperature, it is not mass and velocity. $\endgroup$ – PhysicsDave Oct 4 '18 at 1:11

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