Introduction to Particle Physics
Wednesday, March 27, 2019
Friday, March 22, 2019
Wednesday, March 20, 2019
Tuesday, March 19, 2019
Monday, March 18, 2019
Wednesday, March 13, 2019
Quantization of light 3
The Photoelectric Effect
When light shines on the surface of a metallic substance, electrons in the metal absorb the energy of the light and they can escape from the metal's surface. This is called the photoelectric effect, and it is used to produce the electric current that runs many solar-powered devices. Using the idea that light is a wave with the energy distributed evenly throughout the wave, classical physicists expected that when using very dim light, it would take some time for enough light energy to build up to eject an electron from a metallic surface. WRONG!! Experiments show that if light of a certain frequency can eject electrons from a metal, it makes no difference how dim the light is. There is never a time delay.
In 1905, Albert Einstein came up with the solution. If Max Planck's idea that energy comes in clumps (quanta) is correct, then light must consist of a stream of clumps of energy. Each clump of light energy is called a photon, said Einstein, and each photon has an energy equal to hf (Planck's constant times the frequency of the light). Therefore the energy of light is not evenly distributed along the wave, but is concentrated in the photons. A dimmer light means fewer photons, but simply turning down the light (without changing its frequency) does not alter the energy of an individual photon. So for a specific frequency light, if a single photon has enough energy to eject an electron from a metallic surface, then electrons will always be ejected immediately after the light is turned on and the photons hit the metal.
SUMMARY
According to classical wave theory,
- Intensity of a wave is the energy incident per unit area per unit time.
- Energy carried by an electromagnetic wave is proportional to the square of the amplitude of the wave.
Classical wave theory cannot explain the first 3 observations of photoelectric effect.
1. Existence of the threshold frequency
1. Existence of the threshold frequency
- Since energy of the wave is dependent on the square of its amplitude, the classical wave theory predicts that if sufficiently intense light is used, the electrons would absorb enough energy to escape. There should not be any threshold frequency.
2. Almost immediate emission of photoelectrons
- Based on classical wave theory, electrons require a period of time before sufficient energy is absorbed for it to escape from the metal. Accordingly, a dim light after some delay would transfer sufficient energy to the electrons for ejection, whereas a very bright light would eject electrons after a short while. However, this did not happen in photoelectric effect.
3. The independence of kinetic energy of photoelectron on intensity and the dependence on frequency
- According to classical wave theory, if light of higher intensity is used, the kinetic energy of an ejected electron can be increased. This is because the greater the intensity, the larger the energy of the light wave striking the metal surface, so electrons are ejected with greater kinetic energy. However, it cannot explain why maximum kinetic energy is dependent on the frequency and independent of intensity.
Monday, March 11, 2019
Wednesday, March 6, 2019
He'Story'/ Modern Physics
The Era of Classical Physics, and Cracks in its Foundations (Antiquity-1899)
“Old quantum theory” (1900-1924)
Built on the work of Planck, Einstein, Bohr, and Sommerfeld, and others, “old quantum theory” was never fleshed-out as a fully complete or self-consistent theory, but provided a mechanism for performing calculations involving simple systems such as the hydrogen atom. This era also witnessed the birth of the Special and General Theory of Relativity.
The Birth of Modern Quantum Mechanics (1924-1941)
In this period, quantum theory was mathematically formalized.
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