Experiment 15: Planck's Constant From and LED

Introduction: The purpose of this activity is to experimentally determine the value of Planck's Constant using a diffraction grating and a variety of LED's

Activity One: The energy given to an electron under a a potential difference is E=qV where q is the charge of an electron and V is the voltage potential difference. The energy of an electron also is (h•c)/lambda where h is Planck's constant, c is the speed of light, and lambda is the wavelength of the light source. Setting them equal to each other, we get q•V=(h•c)÷lambda. Solving for h, we get h=(q•v•lambda)÷c.

Activity Two: Procedure: We attached a resistor to the positive of a voltage source set at 3 volts and one end of and LED. We then attached the Other end of the LED to the negative of a voltage source. We then set up a two meter stick with a diffraction grating at the end opposed to the LED and a one meter stick perpendicular to the first.

We looked through the diffraction grating and marked on the one meter stick where the spectrum began and ended. We did this for all the colored LED's and noted the voltage.

Data and Analysis: Here is the data collected on all of the different colors of LED's. We used the same formula from light and spectra (lambda = (d•D)÷√(L^2–D^2)) to find the wavelength of all the LED's. The value of d is given as a difference because the spectrum is not just a line, rather a smear. All of our group members looked through the grating to give 3 trials for every color

The colored LED's only displayed their color when looking through the diffraction grating for the most part. As observed in the picture, blue displays mostly blue a slight bit of green and green displays mainly green a small amount of blue. Yellow, however, displays a little red, orange and green on top of the yellow. The most likely explanation for this is the LED's are not pure forms of their color. In the case of yellow, the shade of yellow is emitting a larger portion of the visible spectrum than just yellow but the majority of the light emitted is yellow. Of course, white emits the entire spectrum. The LED with the largest error is yellow. This is due to the width of the spectrum displayed as just discussed. Below are the data and calculations for lambda and Planck's constant as well as the uncertainty associated. It was planned that we would graph Voltage versus 1/lambda, however this gave a split graph and threw of the linear fit of the slope, which would give Planck's constant. To correct this, we found the value of Planck's constant using the formula from activity one for every color and averaged them. This gave us a more accurate value for Planck's constant.

Questions: We did not see a relationship between wavelength other than green and blue used more voltage than yellow and red. This makes sense because those are more high energy wavelengths. This can be applied to sunset being red and the sky being blue. The sunset will be red because red is a less energetic wavelength and will be refracted by the atmosphere last.