wave propagation in free space

The answer is that the electromagnetic wave (you can imagine it as moving charges) causes that a current (other moving charges) is generated in the conductor plane surface (as well as in a depth of a dielectric or lossy material) which in turn radiates another wave back into space.
Depending upon the angle of incidence, surface current phasing causes that in a good conductor the reflected wave travels along a new line having its angle identical to the incidence angle but to the other side to the normal to the surface.
Lossy materials cause certain delay to the mentioned current phasing, so the angles differ. Dielectric materials split the "new" waves into more than one; some are reflected, others continue under diffent angles through the dielectric material.
Snelli's Law originally defined for light shows the above mentioned angles as functions of the index of refraction; at microwaves, we prefer to use permittivity. Both quantities are complex, so the "new" waves originated by the original wave interacting with materials can be complex, too.

For crying out loud do some work for yourself... this is your fourth or fifth post of questions that you could easily answer yourself with google or, heaven forbid, a trip to a library.

Have Fun

Why should students spend time to learn about electronics if the forum members here are willing to do all their homework?

Reading this thread, there is something that just came into my mind:
Several years ago - at the beginning of my membership to another electronic-oriented forum - I did open a new thread with a provocative question "Is this forum dangerous"?
The background of my question was the following:
I was (and up to a certain degree still I am) afraid that the opportunity to ask simple and basic questions (and expecting reliable answers) could prevent beginners from doing their own research activities - primarily reading/evaluating relevant book chapters.
I think, this thread is an excellent example.

I have been often disappointed by frequent questions on basic problems. Yes, I agree that young people should study first, then ask questions. But I can see that schools grow poorer and most people are naive or lazy to study.
Nevertheless I apologize to others; I think a short explanation may sometimes stimulate the requester to study.