Photoelectric Emission.
When an ElectroMagnetic Radiation source such as Ultra Violet Light inc onto the cap of an electroscope as shown in the diagram. When the uv light falls on the zinc the leaves are seen to collapse. This can be explained as follows. When the uv light strikes the Zinc they transfer energy to it and the electrons on the surface takes up this energy and they leave the surface. This will cause the Zinc to become positively charged. This in turn will attract the negative charges from the electroscope and so the leaves will collapse, leaving the electroscope uncharged.
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Here we see the emission of electrons when the UV light strikes the surface of the Zinc metal |
If the Zinc was covered with glass, then the electroscope would stay as it is as the glass will block any uv light from getting in.(Remember it is not possible to get a suntan inside a glass house).
The emitted electrons will require a certain minimium energy before they will move to the surface of the Zinc. This energy can be calculated from the following equation.
E = hf
where E is the energy, h is Planck's Constant and f is the frequency of the uv light.
If the energy supplied by the uv light will cause the electrons to move away from the surface then this is calculated from the following.
E = hf + K.E.
where K.E. is the Kinetic energy of the electrons emitted.
hf in this case is the works function,þ, and is calculated from
þ = hfo
Where fo is the Threshold Frequency
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