# Measuring Plank's constant with LED's

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?

mfb
Mentor
The speed of light is the speed of light in vacuum (or air, does not matter), as you measure the wavelength there.

Using a red Led for example, which has a maximum wavelength of about 700nm, and a turn on voltage of about 1 volt
That looks suspicious. An electron with an energy of 1eV cannot produce a 700nm-photon. Either there are 2-electron processes happening of something else is wrong.
Are you sure you get light with a voltage of just 1V?

Thanks. I will check the voltage again more carefully. But even with 1.5 volts for a red LED, the wavelength at the shotch on sholder would have to be 800nm. Are we able to see 800nm?

Just looked agian at your answer mfb. Why does the speed have to be where I measure the wavelength? The colour of the light stays the same in glass or plastic doesn't it? Is the color of light dependent on the frequency or wavelength? I am confused..

mfb
Mentor
The fundamental relation (via the Planck constant) is between energy and frequency. The frequency is the same everywhere, but hard to measure. If you measure the wavelength in some medium (or vacuum), you have to use the local speed of light to convert it to the right frequency.

800nm is somewhere at the edge of the visibility - intense laser light can be visible, seeing an LED would probably require a very dark room and a bright LED.