This simple but clever idea is written for those who start seeds in a windowsill, or for those who start them under grow lights. The principal is the same for both.
First let's look at what we can do about starting plants in the windowsill.
Seedlings started in a window sill are going to be long and leggy and lean very strongly in the direction of the sun. They are restricted in the amount of light they receive in three ways and therefore are drawn toward the one diminished light source they have.
If we place a reflective surface behind the plant tray (on the indoor side) so that whatever sunlight comes in gets reflected back toward the plants, two things will happen. Both will be good for your plants.
The first thing that occurs is that there will be nearly as much light coming at the plants from behind (and possibly above) as there is from through the window. This "balanced" lighting will cause your plants to grow upright and not lean "toward the sun".
The second thing that obviously happens is that your plants will get almost twice as much light. This will help keep them from growing tall and spindly from searching for more light. By "feeding" them a more complete dose, they should grow more normally.
I recommend a reflective surface that comes straight up the back for several inches and then curves gently toward the window. If rigid materials are used, straight angles will suffice. {See this drawing page}
The very same principals will apply when growing out seedlings under an artificial light source. Setting up mirrored surfaces on at least one side (if not all four) will send more light to your young plants and help prevent them from stretching and growing up leggy. You could even line the underside of your fluorescent light fixture with reflective material as well. The only thing you will have to watch is that you don't create such a complete reflective "enclosure" that you restrict air from circulating around your plants. Reflective strips (slats) would help prevent this problem.
The more reflective your material is, the more effective it will be, but any improvement . .. is indeed an improvement!
Aluminum foil would work well and is easily formed to fit any shape or contour.
Mylar is even more reflective, but will only curve one direction.
Mirrors (glass or poly) work the best but of course are usually quite rigid and harder to size.
Incidentally, the inside surface of a large Doritos bag works rather well if you want to jump on the re-use ticket.
