1-6: Photoelectric Effect
Although Albert Einstein is most famous for E = mc2 and for his work describing relativity in mechanics,
his Nobel Prize was for understanding a very simple experiment. It was long understood that if you
directed light of a certain wavelength at a piece of metal, it would emit electrons. In classical theory, the
energy of the light was thought to be based on its intensity and not its frequency. However, the results of
the photoelectric effect contradicted classical theory. Inconsistencies led Einstein to suggest that we need
to think of light as being composed of particles (photons) and not just as waves. In this experiment, you
will reproduce a photoelectric experiment and show that the energy (E) of a photon of light is related to
its frequency and not its intensity.
1. Start Virtual ChemLab, select Atomic Theory, and then select Photoelectric Effect from the list of
assignments. The lab will open in the Quantum laboratory.
2. What source is used in this experiment and what does it do? It is a laser and it emits a directed beam of
light at a specific frequency and intensity.
At what intensity is the laser set? 1 nW
At what wavelength is the laser set? 400. nm
Record the wavelength (in nm) in the data table on the following page. Calculate the frequency (in
Hz) and the energy (in J) using c λv and E hν where c 2.998 10 8 ms-1 and h 6.626 10 34
Js. Also record the color of the light by clicking on the Spectrum Chart (just behind the laser); the
marker indicates what color is represented by the wavelength selected.
Which metal foil is used in this experiment? Sodium
What detector is used in this experiment and what does it measure? Phosphor screen.
Detects electrons emitted from Na
Turn on the detector by clicking on the red/green light switch.
What does the signal on the phosphor screen indicate about the laser light shining on the sodium
foil? That it is knocking off electrons from the sheet of sodium
3. Decrease the Intensity to 1 photon/second, how does the signal change? There is a blinking dim signal
Increase the Intensity to 1kW, how does the signal change? The signal increases slightly in intensity
Change the Intensity back to 1 nW and increase the Wavelength to 600 nm.
What do you observe? Record the wavelength in the data table. The signal disappears
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Determine the maximum wavelength at which emission of electrons occurs in the metal. 450 nm
What is the difference between intensity and wavelength?
Intensity is the number of photons, wavelength is the distance between each successive peak of the electromagnetic wave
Which matters in the formation of photoelectrons: intensity or wavelength? Wavelength
Data Table
wavelength (nm) frequency (1/s) energy (J) light color
400. nm 7.50e14 4.97e-19 Violet
600. nm 5.00e14 3.31e-19 Orange
450. nm 6.66e14 4.41e-19 Dark blue
4. Click inside the Stockroom to enter the stockroom. Click on the clipboard and select the preset
experiment called Photoelectric Effect (Bolometer). Click on the green Return to Lab arrow to return
to the laboratory. The intensity of the laser will be set at 1 nW and the wavelength at 400 nm. The
detector used in this experiment is a bolometer and will be automatically turned on. This instrument
measures the kinetic energy of electrons. You should see a green peak on the bolometer detection
screen. The intensity or height of the signal corresponds to the number of electrons being emitted
from the metal, and the x-axis is the kinetic energy of the electrons. Zoom in on the peak by clicking
and dragging from the left of the peak to the right.-
5. Increase and decrease the Intensity, what do you observe? Number of electrons is proportional to intensity
Increase and decrease the Wavelength, what do you observe? Kinetic energy of each electron is inversely
proportional to the wavelength of light
What is the maximum wavelength that ejects electrons from the sodium metal?
Based on this experiment, explain why violet light causes photoemission of electrons but orange light
does not. The energy associated with a single photon of orange light is not enough to knock electrons free.
A single violet photon does have enough energy
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