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 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:
- Identify the angle for which the spectral lines are visible;
- 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
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:
- 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.
- 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.