I actually just copied and pasted your first sentence into google, it took me about 1.5 seconds. Clicked on a link, and this is what I found within like 5 seconds of reading your post.
Many people get mirrors or tin foil and line their grow area
with it. This is NOT good. MIRRORS ABSORB LIGHT. This
means that only a small amount of your light is reflected back towards
your plant. What you need is something white. White is the best color
for reflecting light, period. Not shiny glossy white, just plain ‘flat’
white. A white wall will reflect more light than a mirror will. Many
growers like to paint the walls of their grow ‘flat’ white. This helps a
lot. Other people like to line their grow area with the back end of tin
foil wrapping (the white side). This is okay, but make sure that you use
the white side and not the shiny side.
Cmon man, it's not that difficult to figure out how google works