This question already has an answer here:
- How is light affected by gravity? 3 answers
Light is affected by gravity which means light has got mass. Quantum theory tells us that light consists of discrete quanta of energy, which we call photons. These photons have a rest mass of zero (as a consequence of special relativity), but a "moving photon" (I don't know whether that notion is stupid) has got some mass.
According to Planck, a photon travelling with a frequency ϑ will be associated with an energy
$$ E=hϑ. $$
Since the same photon is travelling at the speed of light, it should hold valid for relativity also. In relativistic theory, the energy of a photon is given by
$$ E=pc $$ for a photon $p=mc$ where $m$ is the relativistic mass of the photon. Hence $E=mc^2$
Comparing both equations, $$ hϑ=mc^2$$ or
$$m=hϑ/c^2$$ which means the mass of a photon is dependent on its frequency. This is the cause of redshift of light in a gravitational field. But how frequency account for mass? Is it a direct evidence that light and matter are not distinct entities of the universe, but matter is nothing but condensed energy?