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Quantum Phenomena Notes

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Unit 1: 3 Quantum phenomena

PHOTOELECTRICITY Observations of the photoelectric effect;

• Emission starts the instant the radiation falls on the surface

• Frequency of the radiation must be above a certain frequency, known as the threshold frequency, fo

• Increasing the frequency of the photons increases the photoelectron's kinetic energy.

• Photoelectrons emitted per second is proportional to the intensity (or brightness) of the radiation.

• The intensity has no effect on the energy of the emitted photoelectrons.

Contradicts the wave theory of light; WAVE THEORY STATES THAT INTENSITY DETERMINES THE ENERGY OF WAVES, NOT FREQUENCY
-If the intensity was very low, you would not expect any conduction electrons to be emitted.
-If the intensity was increased, the energy is increased, therefore you would expect electrons to start to be emitted. (Intensity is brightness (for light) and is the energy per unit area per unit time)
-Changing the frequency of the light should NOT affect the emission of electrons because the frequency of a waves does not affect its energy.

Einstein's explanation for Photoelectric effect;

• Light could be considered to consist of packets of quanta (photons) of energy; E = hf;

> h is Planck's constant (6.63 x 10-34 Js)
> f is the frequency of the electromagnetic radiation.

• The energy of the photon would then provide the energy required to overcome the work function of the metal surface, with any excess going into maximum kinetic energy with which the electron would leave the surface.

E = ϕ + Ek

hf = ϕ +

hc = ϕ +
λ

1/

1/

2mv

2mv

2

2

Where:
>hf is the energy of the photon (in J)
> ϕ is the work function of the metal surface (in J)
>1/2mv2 is the maximum energy of the emitted photoelectron (in J) REMEMBER:
-m is the mass of an electron (9.11 x 10-31)
-c is the speed of light (3 x 108) PHYSICS AS NOTES

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