Perfectly reflective surfaces

I was thinking, If somehow we could create a perfectly reflective surface, create a sphere with it, couldnt we technically trap LIGHT inside of it? If a surface was perfectly reflective could it still be used as a solar sail since all the energy of the light is left completely the same after it is reflected?

I just got out of highschool so I really have no idea of the theories or anything behind this, so could someone with more knowlege explain if this is possible?

I am more interested in what kind of material one would use to create such a mirror. I think mercury would be a good substance to use. any thoughts?