Ive heard using the light is more of a selling point than actually making a difference. If Im not wrong there should be an article about it somewhere where they tested the whitening capabilities of certain agents with and without the light.
Here are some related articles:
Sulieman, M., Reese, J.S., & Addy, M. (2006) Surface and puplal chamber temperature rises during tooth bleaching using a diode laser: a study in vitro. British Dental Journal, 200, 631-634.
Luk, K., Tam, L., & Hubert, M. (2004). Effect of light energy on peroxide tooth bleaching. The Journal of the American Dental Association, 135, 194-201.
Matis, B.A., Gaiao, U., Blackman, D., Schultz, F.A., & Eckert, M.A.S. (1999). In vivo degradation of bleaching gel used in whitening teeth. Journal of the American Dental Association, 130, 227-235.
Sulieman, M., Macdonald, E., Rees, J.S., Newcombe, R.G. & Addy, M. (2006). Tooth bleaching by different concentrations of carbamide peroxide and hydrogen peroxide whitening strips: an in vitro study. Journal of Esthetic and Restorative Dentistry, 18, 93-101.
Naik, S., Tredwin, C.J., & Scully, C. (2006). Hydrogen peroxide tooth-whitening (bleaching): review of safety in relation to possible carcinogenesis. Oral Oncology, 42, 668-374.
Kihn, P.W., Barnes, D.M., & Romberg, E. (2000). A clinical evaluation of 10 percent vs. 15 percent carbamide peroxide tooth whitening agents. The Journal of the American Dental Association, 131, 1478-1484.