Booy, great question...
Many people believe that the roots should not be exposed to lights.. and some even say that the plants will die if the roots are exposed to lights/... But many a seasoned farmers have witnessed first hand, plants that grow very well whilst the roots are exposed to light... aye caramba... so what's the answer?
The skinny--- When water and light mix.. anything can grow... this is the main reason YOU DON'T (DO NOT) want to expose your soil, reservoirs, water or your roots to light... fungus, algae, molds etcetera can/will free load off of your grow...
when algae grows in water (your hydro reservoir) it will take all the O2 from roots and the plants will DIE.. when gardeners use clear containers (platic cups) to grow plants, the moist soil exposed to light encourages mold and fungus growth which eventually kills your roots...
It is just NOT worth the risk... LIGHT is good for your plant.. but not for your MEDIUM...
iloveyou