The Photo Electric Effect

By Ash =]

Good Grief… what is that ?The photo electric effect is the process of ejecting

an electron from a metal atom with the use of a photon.

In order for the photon to eject the electron it must have sufficient amount of energy that exceeds the ‘work function’ of that metal. What ever is left over is converted into kinetic energy as the electron leaves the atom.

The energy of the photon is related to its frequency.

hf = Ø + Ek

Still don’t get it?

This is Sodium!

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Key facts:

Proud owner of 11 electrons, 11 protons and 11 neutrons.

Sodium’s favourite number is also 11 =]

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Sodium’s electrons are arranged into shells.

In the inner shell there are a couple

In the middle shell there are 8 more

And finally 1 lone electron in the outer shell

This is Sodium!

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It is this ‘loner’ that we are interested in, as this will be the electron which we will be ejecting.

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In order to do this we need to fire a photon at it with enough energy to eject it from the atom

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The ‘work function’, (which is the energy required to remove an electron) is 3.65 x 10 J

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So what frequency photon do we need?

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The energy of a photon is calculated by:

E = hf

The frequency can be calculated by rearranging this formula.

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f = E h

…and plug in the numbers

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f = 3.65 x 10 J 6.63x10

f = 5.5 x 10 HZ14

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-34

The higher the frequency, of the colour, the closer together the waves of energy are.

Higher frequency colours are: violet and indigoMedium frequency colours are: Blue, green and yellowlower frequency colours are: orange and red.

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5.5 x 10 HZis about here in yellow

4.3 x 10 HZ 7.5 x 10 HZ14 14

This is the visible light Spectrum:

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This means that we need at least yellow to eject electrons.

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If we fire a red light photon, it does not have enough energy as its frequency is too low.

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If we fire a yellow light photon it has sufficient energy and ejects the electron with a little Ek

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If we fire a blue light photon it has more than enough energy and eject the electron making the electron travel much faster.

To calculate the Ek electron once ejected when a blue light photon of frequency 7.7 x 10 HZ has been used

we do:

Ek = hf – Ø

1.46 x 10 J = [(6.63x10 )x(7.7 x 10 HZ)] – (3.65 x 10 J)-19

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Got it ?

… but what is it used for?

When you have removed an electron from any metal it becomes an ion.

Because the electron had a negative charge, this ion is positively charged.

This charge produced makes a current due to the flowing of electrons.

This small current can be used to create light dependant components that activate when light is shined on them.

This includes search lights and gamma light systems used to picture inside patients.

Cheers for listening =]