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V-A characteristics of the LED
(approximate determination of Planck's constant)

Work tasks and measurement procedure

Work task:

  1. Measure point to point, volt-ampere characteristics of the LEDs of different colors (red, yellow, green, blue and white).

  2. Plot the obtained values and qualitatively compare the shape of the obtained dependencies. Explain the similarities V-A characteristic white and blue LED.

  3. Fitting the linear part of the V-A characteristics determine the threshold voltage U* of each LED.

  4. Get the value of slope of the linear part of V-A characteristics using linear regression (eg using spreadsheet) and specify dynamic (differential) resistance of the LED.

  5. Determine the wavelength of light emitted from the LED's (except white) from values of the threshold voltage and compare this with the value given by the manufacturer.


  1. Determine (at least of the order estimate) the value of Planck's constant from the value of the threshold voltage of each LED and knowledge of the emitted wavelengths.

Measurement procedure

  1. Start the remote experiment CEcIL.

  2. Choose a measured LED (see description of controls).

  3. Use the slider (PC) or buttons + & (tablet) gradually set various values of the electric voltage on the LED.

  4. Read the values of voltage and electric current and write to the table.

  5. Monitor continually the measured values in the chart and in the case of larger gaps between the points of depending measure missing parts.

  6. Follow the same procedure for all LED. After measuring each LED can save the obtained experimental data on the disk of your PC.

  7. Exit remote task after measuring the required number of data (and store all files of the experimental values!).

  8. Open the selected file of experimental data in a spreadsheet.

  9. Plot the dependence of the electrical current through the LED is connected to an electrical voltage to the chart. Check the shape obtained dependence to the expected shape.

  10. Select the points of the linear part of the dependence and interleaving by a linear regression including display of its equation. Get the threshold voltage U* by determining the intersection with the horizontal axis (graphically and numerically).

  11. Slope of the regression line corresponds to the dynamic (differential) resistance LED.

  12. All V-A characteristics can be compared together by plotting to common graph.

Approximate determination of Planck's constant
  1. Use the values of the threshold voltage of a formula (4)

  2. You will get the following expression (through formula (4)), which you can calculate the wavelength:

λ =   h·c 

  1. Compare the values of wavelengths with the following table wavelength LED used in this experiment. Consider the deviation measurement!

 Color  λ / nm 
 red  640
 yellow  590
 green  568
 blue  470


Determining the wavelength of the emitted light
  1. Also here you will know the value of the threshold voltage U* and formula (4), which you can edit in the this form, from which we determine the value of Planck's constant:

 h =   e·U*·λ   

  1. Calculate the value of Planck's constant for all monochrome LED and average the obtained values.

  2. Compare the determined value of Planck's constant with a theoretical value. Rather, it is an estimate of the order of magnitude (with respect to the configuration of the experiment), but it should correspond.