Determination of Planck's Constant

Description of the Experiment

In this experiment is possible study the photoelectric effect and calculate Planck's constant by using 5 different coloured leds and a photoelectric cell.

Experimental Apparatus

Figure 1: Led's spectrum.

The photoelectric cell is from the apparatus PASCO AP-9368. It works like a capacitor where an electric conductor target emits photoelectrons when excited by light. The potential of the photocell capacitor armature will increase with the emitted photoelectron accumulation. After reaching a certain voltage, the stop potential, no more photoelectrons will have enough energy to reach the second armature. This voltage will depend on the wavelength of the incident light (photon energy).

After each experiment the photocell is connected to ground to discharge it.

The leds have different quantum efficiency leading to different intensities for chosen bright leading to different times to reach the stop potential.

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

Protocol

The number of photoelectrons emitted will increase with the intensity of light. (corpuscular behaviour 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. Repeat step 2 for different intensities.

Example of a table

Color #1 __________(name)	Intensity (%)	Stop Potential (V)	Time (s)
	100
	80
	60
	40
	20

Figure 2: Potential vs. Peaks Frequency of the spectrum

The photoelectrons 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 colour leds.
2. Draw a graphic of Stop Potential vs Frequency. Fit it to \( V = \frac{h}{e} \nu - \frac{W_0}{e} \) and obtain Planck's constant.

Example of a table

Colour (name)	Frequency (Hz)	Stop Potential (V)