Planck's Constant Determination

Description of the Experiment

The purpose of this experiment is to study the photoelectric effect in metal and the determination of Plank'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

Under construction.

Protocol

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

Figure 1: Graph with the phototransistor's output.

Table 1 – Caracteristic wavelengths for the mercuty lamp

Colour	Frequency (Hz)	Approximate wavelength (nm)
Violeta longínquo	8.22x1014	365
Violeta	7.41x1014	405
Azul	6.88x1014	436
Verde	5.49x1014	546
Amarelo	5.19x1014	578

Obs:

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.