Experiment 14 Planck's Constant from an LED

Purpose:

the purpose of this experiment is to determine the Planck's Constant through the experiment, furthemore determine the voltage, wavelength, Planck's Constant relationship.

Equipment:
Assortment of LEDs(red, Green, Yellow, Blue), 390ohm resistor, Voltage Supplies, Alligator Clips, Extra wire to extend battery clips, optics stuff from previous week, voltage probes.

Experiment and Data Collection:

first of all, set up two meter, fix them in a right angle, and fix the LED light at the corner of the two meter, at other end of then long meter, put a alligator, to watch the LED light ,find the diffraction light. totally, we change 4 color, green, orange, red and blue.

we measure the length from the small diffraction grating to the meter, is L = 191.85 cm, and the diffraction grating is 300 slice per mm, so we get the d = 0.001m/300 = 3.33*10^-6 m, then we start the experiment, one student watch the LED throught diffraction grating, to determine the diffraction fringe position, the other student stand close to the meter to read the bright fringe positon on the ruler follow the first student. and the third student use voltage ammeter to read the voltage of the LED.

data table

Color	D/m	Voltage/V
Green	0.318	2.48
Orange	0.338	1.88
Red	0.363	1.81
Blue	0.284	2.58

then, we use equation wavelength = Dd/sqrt(D^2 +d^2) to find out the wavelength of different color, use equation f = c/wavelength to find out the frequency of different color, use E = e*V to find out the energy of different LED light.

	Wavelength/m	Frequency/Hz	Energy/eV
Orange	5.78*10^-7	5.19*10^14	2.48
Red	6.19*10^-7	4.85*10^14	1.88
Blue	4.48*10^-7	6.70*10^-14	1.81
Green	5.45*10^-7	5.50*10^14	2.58

then we make a graph, but our graph are not on a straight line., but roughly we get a slope is 1.98*10^-7 meter, so we have the Planck's constant.