Do photons have mass and if not then how is it affected by gravity then why and how does it slow down when passing through a medium and then instantly accelerate back to light speed?

Answer 1

Part 1: General relativity has gravity act on photons not directly (as with objects with mass) but by bending the space&time around the object. The photons are unaffected. The path they take is curved, so they appear to bend. Sort of like how you can walk in a "straight line" on earth, but eventually appear where you started. Because the earth is curved, your path is curved, it has nothing to do with how much mass you have.

Now there is a way to describe reflection and refraction using only photons, and that process is described below by other posters:

The process of reflection/refraction is really a process of absorption/emission on a really small level. When electrons, say those of silicon atoms, are bombarded with photons, they absorb their energy, and, quite temporarily, the photon ceases to be observable as light.

The electron then immediately emits a photon in the opposite direction, producing the illusion of light moving more slowly; however, we know that the speed of light is constant in every possible scenario, even outside a vacuum. This just happens to be one of the strange consequences of the quantized state of matter at the sub-atomic level.

An atom is a system which wants to exist in its ground state. It does not want to absorb the energy of a photon and thus, it ejects the energy in the form of a photon (the energy difference as a result of the "collision" is manifested in a slight change in the wavelength/frequency of the emitted photon)

Very good answer above, for further information on electron reflection/diffraction, Richard Feynman has some excellent free video lectures on QED that were intended for the layperson.

Answer2:

Photons have mass and the concept of rest mass is false.

Daring to challenge General Relativity, Photon's have mass W = hf = pc = mcc, thus m=hf/c2= h/cw and thus is affected by gravity.

Einstein's Deflection 1.7" is the result Gravity tan D= .5gt2/150Gm = .5 x9.8 x150G/c2=8.16666E-6 ,

where t= 150Gm/c and g= 9,8 earth's gravity.

Answer 3: There is a reason why gamma, X-ray, Bharat Radiation, and light photons attract towards Earth. Both energy and matter constitute three components: electric, magnetic, and gravitational. In clear words, since photon's gravitational component facilitates attraction towards Earth.