Photoelectric Effect
(1) Explain the results of experiments involving the photoelectric effect in terms of the photon model of light.
Photoelectric Effect
Scientists noticed that negatively charged objects would emit electrons under light, in certain conditions. This was called the photoelectric effect. The photoelectric effect is the emission of electrons from a material when light of a certain frequency shines on it.
Photoelectric Effect: The emission of electrons from a material when light of a certain frequency shines on it.
Experimental Results
| Experiment | Procedure | Results |
|---|---|---|
| Millikan | Light is blasted at an emitter plate of different metals at different frequencies and intensities, which releases electrons. Electrons fly through the vacuum and hit a collector plate. The electrons can be measured to find the current. Alternatively, the collector plate can be charged to repel elec- trons, which determines stopping voltage, and by extension, the max KE. | Stopping Voltage was found to be constant for each metal. The max KE of the electrons was found to be proportional to the frequency of the light. The max KE of the electrons was found to be independent of the intensity of the light. Frequency and current have a curved quarter circle relationship. |
Einstein
- Albert Einstein proposed that light is quantized, meaning it is made up of particles called photons.
- Photons have energy, and when they hit an electron, they transfer their energy to the electron.
- If the energy of the photon is greater than the work function of the metal, the electron will be ejected.
- The work function is the minimum energy required to remove an electron from the surface of a metal.
- The energy of the photon is given by the equation: E = hf where E is the energy of the photon, h is Planck’s constant, and f is the frequency of the light.
- The energy of the ejected electron is given by the equation: KE = hf - W0 where KE is the kinetic energy of the electron, h is Planck’s constant, f is the frequency of the light, and W0 is the work function of the metal.
Wave & Particle Models of Light
| Situation | Model | Explanation |
|---|---|---|
| Laser shown through 2 slits results in many dark and light bands on a screen | Wave | Light is a wave and the bands are due to constructive and destructive interference. |
| Laser beams do not bounce when they intersect | Wave | Waves add amplitudes as they pass each other but do not impact each other. |
| Reflection angle matches angle of incidence | Particle OR Wave | Both photons and wave models are consistent. |
| Metal has a characteristic work function | Particle | Photons must have a certain energy to eject electrons. |
Electroscope Experiment
An electroscope is a device that can detect the presence of charge, by suspending 2 metal leaves inside a beaker/flask, and contains a piece of metal on top. When the electroscope is charged, the leaves will spread apart, since electrons are either repelled or attracted to the metal piece, causing the leaves to have the same charge.
Decharging:
- When UV light is shone onto an electroscope that has zinc on top, the leaves will collapse, since the UV light has enough energy to eject electrons from the metal piece, causing the leaves to lose their charge.
- Decreasing the intensity of UV light will decrease the rate of decharging, however, the decharge still happens.
- Decharging does not work with red or green light because red nor green light have the energy to surpass zinc’s work function.
- Similarly, using UV light to decharge a copper electroscope does not work, because not even UV has the energy to surpass copper’s work function.