Reflection of light (and other forms of electromagnetic radiation) occurs when the waves encounter a surface or other boundary that does not absorb the energy of the radiation and bounces the waves away from the surface.

What makes a reflection?

Reflection is when light bounces off an object. If the surface is smooth and shiny, like glass, water or polished metal, the light will reflect at the same angle as it hit the surface. ... For a smooth surface, reflected light rays travel in the same direction. This is called specular reflection.

What causes reflection in water?

The most fundamental answer is that water reflects light because the wave impedance of water is different than the one of air and the electric and magnetic field must be continuous everywhere in space.