(I tried, but the wikipedia chapter is too complicated for me => trying to simplify here)
> ...it's momentum and energy.
Therefore, if the photon does not have a mass but is still able to have an effect against the target body it impacts with, then the photon must lose energy, right?
[puah - for some reason I cannot reply to the my post's - typothrowaway's - reply...]
> (I tried, but the wikipedia chapter is too complicated for me => trying to simplify here)
I can imagine. That stuff on the wiki is not even in every physics bachelor. It's definitely not for a general audience, but I thought it might help a little :)
> Therefore, if the photon does not have a mass but is still able to have an effect against the target body it impacts with, then the photon must lose energy, right?
Yes, or gain it if it slows down the object, if the object is traveling towards the light.
Or the internal energy of the object is changed. This happens when a photon is absorbed.
Here you can read the relations between a photon's momentum and things like wavelength. Without any mass :)
https://en.wikipedia.org/wiki/Photon_momentum#Physical_prope...
(You need to scroll down a bit for the formulas)
These results come from special relativity(and a bit of quantum mechanics), which you really need if things don't have mass.