A)
As you carefully observe the cathode during the experiment, begin by varying the wavelength/frequency of the incident radiation in order to identify the value at which the emission of electrons from its surface begins. Then, by gradually changing the intensity of the incoming radiation, observe how the number of emitted electrons changes and whether the onset of emission is affected by this variation.
FIGURE 1
FIGURE 2
At the same time, adjust the applied voltage between the anode and the cathode and systematically record the values of the photocurrent on the corresponding graph. In this way, you will obtain the characteristic curves of the photoelectric effect, observing both the saturation region and the stopping potential.
Try to relate the changes you observe in the graph to the physical interpretation of the phenomenon — that is, the role of the intensity and the frequency of the radiation in the emission of electrons.
B)
Next, vary the wavelength in order to determine the threshold wavelength (or equivalently the threshold frequency) for each material, and compare how the stopping potential changes. This will allow you to verify that the intensity mainly affects the number of emitted electrons, whereas their energy is determined by the wavelength (or frequency).
FIGURE 3
FIGURE 4
In this way, you will be able to identify where the classical description fails and understand why Einstein’s photon hypothesis fully explains the experimental data and the shape of the curves.
Key relations of the photoelectric effect
Photoelectric Effect – Key Relations
The energy of a photon is given by:
E = hf = hc/λ
For an electron to be emitted from the
cathode, the photon energy must be at least equal to the work function Φ:
hf ≥ Φ
Einstein’s photoelectric equation is:
Kmax = hf − Φ
where Kmax is the maximum kinetic energy of the emitted electrons.
The maximum kinetic energy is related to
the stopping potential Vs as:
Kmax = eVs
Therefore:
eVs = hf − Φ
The threshold frequency f0 (below which no
electron emission occurs) is:
f0 = Φ/h
The corresponding threshold wavelength λ0
is:
λ0 = hc/Φ
Effect of Intensity (Qualitative)
• The intensity affects the saturation
current (number of electrons emitted per second).
• The intensity does not affect the
stopping potential Vs for a given frequency, since Vs depends on Kmax and
therefore on the frequency.
Qualitative conclusion about intensity
From the curves you will observe:
The intensity affects the saturation current (i.e. how many electrons are emitted per second).
However, it does not affect the stopping potential for a given frequency, because the stopping potential depends on the maximum kinetic energy, which in turn depends only on the frequency of the radiation. (Encyclopedia Britannica)
Αν θέλεις, μπορώ να στο “γυαλίσω” ακόμη περισσότερο για publication (π.χ. IB, conference ή OER template) ή να το μετατρέψω σε Word-ready μορφή με headings, captions και student prompts.
https://photodentro.edu.gr/v/item/ugc/8525/2550
https://www.seilias.gr/images/stories/html5/Bohr.html
https://www.seilias.gr/images/stories/html5/emWave.html
ATOM H2
https://phet.colorado.edu/sims/html/models-of-the-hydrogen-atom/latest/models-of-the-hydrogen-atom_all.html
https://phet.colorado.edu/sims/html/models-of-the-hydrogen-atom/latest/models-of-the-hydrogen-atom_all.html
