I have been trying to do an experiment measuring Plank's constant using LED's of different colors. The result I get is out by a factor of nearly two. I.e. I got about 3.7 x 10^-34 Js . I am not sure what I am doing wrong. Using a red Led for example, which has a maximum wavelength of about 700nm, and a turn on voltage of about 1 volt, I do the following calculation. E = hf. So h = E/f = (1 x 1.6^-19 )/ (3 x 10^8/700 x10^-9) = 3.73 x 10^-34. Should I be using a different value for the speed of light because the light is being first emitted in plastic? Clear plastics typically have refractive indexes close that of glass. Using a value of 2 x 10^8 for plastic still only Πgives planks constant as 5.5 x 10^-34 Js. This leads me to thinking that I have made some other fundamental blunder, but I am not sure what it is. Should the be a factor of pi in there or something?