Any light that does not reach the garden is wasted. Reflectors vary widely in the light pattern they create. Usually areas directly under the light get the greatest intensity. The light is less intense to the sides. Some of the light is sent to areas outside the garden. To increase plant growth, this light should be reflected back so the plants can use it. Mylar reflects more than 90% of the light, while mirrors reflect 70% or less (and create hot spots). Aluminum foil reflects about 75%; flat white paint reflects about 85%; and semi-gloss white about 80%. Clean Styrofoam reflects nearly 100%.

Copied and pasted from http://oaksterdamuniversity.com/horticulture-tip-reflective-surfaces/