Photons have zero rest mass, but light has an energy of $h\nu$, where $\nu$ is the frequency, so this gets added on to the mass of our thing.
Oversimplified explanation: Photons are massless so they can go as fast as they want without needing kinetic energy, but even massless things can store energy (think of the massless springs you studied in high school physics). Energy is the same thing as mass, but we still refer to photons as massless, in the same way that we think of an (imaginary) massless spring as such; we don't like to think of potential energy as _being the object_, just somehow _there_.
This makes more sense if you study physics, in no small part due to the fact that we make it all rigorous (though not to the satisfaction of the mathematicians, I guess...).
Zergy|12 years ago
If so then how does a massless object add weight to an object by being trapped in it.
atondwal|12 years ago
Oversimplified explanation: Photons are massless so they can go as fast as they want without needing kinetic energy, but even massless things can store energy (think of the massless springs you studied in high school physics). Energy is the same thing as mass, but we still refer to photons as massless, in the same way that we think of an (imaginary) massless spring as such; we don't like to think of potential energy as _being the object_, just somehow _there_.
This makes more sense if you study physics, in no small part due to the fact that we make it all rigorous (though not to the satisfaction of the mathematicians, I guess...).