Planck's Constant Determination

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Description of the Experiment

The purpose of this experiment is to study the photoelectric effect in metal and the determination of Planck's constant, since the former is of a quantic nature.

The light created by a mercury bulb is passed through a diffraction net to separate its spectral lines (i.e., the colours of the spectrum, just as a rainbow is a separation of the sunlight) and is used to force the emission of electrons in a photoelectric cell for each colour.

Experimental Apparatus

Led spectrum
Figure 1: Led spectrum.

The photoelectric cell is a PASCO AP-9368, capacitor type cell, where one of the plates emits photoelectrons. These are stored in the other plate, which creates an electric potential between plates (and consequently, between the photocell's terminals). This potential increases with time until a maximum is reached. At this point, the capacitor stops charging (the potential energy is greater than the photoelectron's kinetic energy). For each wavelength, the electric potential will be different.

The photocell's terminals are grounded after measurements are taken to ensure that the capacitor is discharged when the next experiment starts. This allows an accurate determination of how long it takes for the capacitor to charge when considering different radiation intensities (not to be confused with the photoelectron's energy).

Each color's (led) efficiency is different, so the intensity will also be different. This means that the charging time will be influenced by these factors.

Table 1 – Spikes in the Led spectrum
Color Frequency (Hz) Wave lenght (nm) Led spectrum
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


There are two steps to achieve the goals of this experiment:

  1. Identify the angle for which the spectral lines are visible;
  2. Select the line to analyse and the filters to avoid reflected light with different wavelenghts.

To identify the lines, the experiment must first be done in sweep mode. This way, the output of phototransistor will show what are the angles that have light. The order of the peaks is shown in table 1

Graph with the phototransistor's output where the angles for the spectral lines can be identified.
Figure 1: Graph with the phototransistor's output.

Table 1 – Caracteristic wavelengths for the mercury lamp
Colour Frequency (Hz) Approximate wavelength (nm)
Ultra violet 8.22x1014 365
Violet 7.41x1014 405
Blue 6.88x1014 436
Green 5.49x1014 546
Yellow 5.19x1014 578


  1. The experiment will automatically turn the mercury bulb off after 30min. After this happens, there is a 15min waiting period to stabilize the power running through the bulb.
  2. It is important to use the correct filter to avoid the superposition of two colours in the photocell, something that can occur because of diffusion of light.

Plank's Constant Determination

UNDER CONSTRUCTION. By running the experiment in NORMAL mode, the output will be a time-dependent graph of the capacitor's charge, which is determined by the current created by the photocell. The less time is takes to charge the cpacitor, the higher the current will be, which means higher light intensity (as shown in the advanced prtocol).

  1. There are five observable colours (the main, discrete lines that correspond to the transitions between the discrete energy levels of of the Hg atom). Adjust and select the angle as a function of colour.


Advanced Protocol

Under construction.

Classic model for radiation and particle model

Under construction.

Theoretical Principles

Under construction.

Photoelectric Effect

Under construction.

Historical Elements

In 1921, Albert Einstein won the Nobel Physics Prize for his discovery of the law of photoelectric effect.