Determination of Planck's Constant

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Description of the Experiment

In this experiment is possible study the photoelectric effect and calculate Planck's constant. Using 5 colored leds and a photoelectric cell.

Experimental Apparatus

Espectro dos Leds
Figure 1: Led's spectrum.

The photoelectric cell is from the apparatus PASCO AP-9368. It works like a capacitor where an electric conductor emits photoelectrons. The potential of the photocell will increase with the emission of photoelectrons. And the stop potential will depend on the wavelength of the incident light, photoelectric effect.

The photocell is connected to ground to discharge, after each use.

The leds have different spectrum and intensities so the time to reach the stop potencial may vary.

Table 1 – Led's spectrum peaks
Color Frequency (THz) Wavelegth (nm) Espectros dos leds
Blue.ab 638.7 469.70 File:Espectro Azul.ab.txt
Blue 684.6 438.20 File:Espectro Azul.txt
Red 482.2 622.21 File:Espectro Vermelho.txt
Yellow 514.4 583.16 File:Example.txt
Green 530.8 565.22 File:Espectro Verde.txt


The number of photoelectrons emited will increase with the intensity of light. (corpuscular behavior of light)

  1. Choose a led to light up on the photocell
  2. Measure the stopping potential. Take note the time necessary to reach the maximum potential.
  3. Repit step 2 for diferent intensities.
Exemple of a table
Color #1 __________(name) Intensity (%) Stop Potential (V) Time (s)

Constante de Planck
Figure 2: Potential vs. Peaks Frequency of the spectrum

The photoelectron's kinetic energy depend only on the frequency of light. If the frequency of light increase the energy will increase.

  1. Obtain the stop potentials for different color leds.
  2. Make a graphic of Stop Potential vs Frequency and obtain Planck's constant.

Exemple of a table
Color (name) Frequency (Hz) Stop Potencial (V)

Advanced Protocol