if rest mass of photon is zero!! when photon collides with a surface it will lose some energy satisfied by E2-E1 of metal. but at this time during collision photon comes to momentary rest(by some micro seconds)!!!!! does this mean at this time its mass is zero?? if so when it continues its motion where does it regain energy:rolleyes:

It doesn't stop. It gets absorbed by the material and a lower energy photon is then emitted.

if photon is considered particle during motion why don't we consider of it bouncing back after giving some energy!! why do we only consider absorption and re emission

You're not considering basic laws of Physics here. ydoaPs is right. You can't talk about collision of photons in this context either.

if it doent bounce back the why is it called collision?