If light has no mass how can it be deflected by the gravity of a star?

Here is the understanding of photons before Quantum Mechanics started to develop in the early 1920's.

Involving QM in a simple explanation turns the things ugly, because you and I have to deal with probabilities, uncertainties, statistics and the complex mathematical works of De Broglie, Heisenberg, Schrodinger, Feymann, Fermi and 100's of physicists since then.

Many physicists insists in the explanation than links Einstein and Planck equations PLUS Newton Mechanics.

And the key factors are: Light has ENERGY and also has MOMENTUM. With these simple assesments, and having in mind that, for Einstein, mass was explained through its energy and that for Planck, the energy of a "quanta" depends on its frequency, a simple equation appears that relates:

1) Einstein E=mc2, but where m is the relativistic mass (excluding mass at rest).

2) Planck E=kf, where k is the Planck constant and f the frequency of light.

3) Newton p=mv, wich is linear momentum (mass x speed) for a
given mass.

4) Accepting that light has momentum but NO RESTING MASS.


Mixing all them togheter, it's found that:

p = hf/c = mc (light momentum)

then, light mass (relativistic, not inertial) is:

m = hf/c2 ,and this equation tells the mass of a photon.

Mass of a photon grows linearly with the frequency of light.

I don't like it, but is what is told by several scientist.

So, having matter, light is subjected to gravitational attraction. What is said is that force is so small than can't be measured properly in outer space.

Yes it can be measured the attraction force of a celestial body over the light (a different matter).

Here is a nice video: