How do Dual layer DVD work?

Answer: How dual layer CD drives workCheck out this website: http://computer.howstuffworks.com/cd-burner.htm

The main difference between a single layer and dual layer CD is that a dual layer CD or DVD has two layers of film on which data can be written to. One layer (the bottom one) is semi-silvered so that depending on the intensity of the laser, data can be read on each surface. The laser adjusts its intensity accordingly to which layer it want to read/write from.