Quantum physics: The photoelectric effect
What is the photoelectric effect?
The photoelectric effect is when a clean metal surface emits electrons after light, especially ultraviolet, has been shone on at it. This happens because of a process called photoelectric emission whereby a single photon package of electromagnetic energy, called a quantum, is absorbed by metal surfaces. This energy is then transferred to one single electron which can then be released by the metal and that released electron is known as a photoelectron. The effect was first absorbed by German physicist Heinrich Hertz in 1887 and another discovery he discovered said that metal surfaces where also more likely to emit sparks if they had also been illuminated.
A current can measured when the plate has a ultraviolet light source applied during it as the photoelectrons have an negative charge and so are attached to the positively charged plate. This means if the photoelectrons have enough Kinect energy they will be able to reach the negative plate and a current will be recorded.
The photoelectric effect is demonstrated using something called a charged golf-leaf electroscope and conducted by putting a piece a clean piece of zinc on the cap of the electroscope which is negativity charged.(see below an example taken form www.a-levelphysicstutor.com )
Note the zinc would be placed on top of the blue strip
When the light is switched on the negativity charged end of electroscope and the negatively charged gold leaf repel each other. By bringing the Ultra violet light close to the zinc the gold leaf start to move back towards the stem and by moving the lamp backwards the leaf as a result sops falling. From this it makes it clear to us that the ultraviolet light is causing the emission from the zinc of the electrons. A sophisticated electrometer will measure how much energy is being emitted from the electrons during the process.
Einstein’s theory was that the photons energy