# Is mass of light constant

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1. Jun 25, 2015

### jeevesh

is mass of light constant

2. Jun 25, 2015

### Staff: Mentor

As asked, this question doesn't make much sense - it's not at all clear what you mean by "the mass of light". Can you try to be more specific?

But before you do, try searching this forum and google for "light mass" and see what you find.

3. Jun 25, 2015

### rootone

Last edited: Jun 25, 2015
4. Jun 25, 2015

### Noctisdark

To make it simpler, light made up of little particle called photons, the photons have energy E = hc/λ, according to special relativity E = Mc^2, although they don't have rest mass, they have relativistic mass (another term of energy), if you want to know how einstein got in this he assumed that an object radiated some light, so he outputted some energy E and by conservation of energy he has lost some energy E and after some calculation he discovered that the radiated energy E is equal to the change of mass times c squared, E = Mc^2 = hc/λ so Mc = h/λ, but Mc looks like momentum that's when einstein suggested that p = h/λ, we write p but not Mc because photons carry no mass but energy and energy is equivalent to mass,

Last edited by a moderator: Jun 26, 2015
5. Jun 26, 2015

### Staff: Mentor

Much of what you say in this post is wrong. The relationship between light, which is electromagnetic radiation governed by Maxwell's equations, and photons is much more complicated than you suggest - you cannot think of light as a stream of little particles. Nor do photons have relativistic mass, as the definition of relativistic mass (check out https://www.physicsforums.com/threads/what-is-relativistic-mass-and-why-it-is-not-used-much.796527/ [Broken]) is defined as $\gamma{m}_0$ where $m_0$ is the rest mass - and of course the rest mass of a photon is zero, so the relativistic mass also comes out to be zero.

Last edited by a moderator: May 7, 2017
6. Jun 26, 2015

### Staff: Mentor

Several posts back we asked the original poster to clarify his question. I suggest that we hold further discussion until he does that.

("suggest that we hold further discussion until..." is mentor-speak for "don't post again unless you're very sure that you have something new, important, and correct to say").