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THE UV CATASTROPHE
Theoretical ­ Predicted that at shorter
wavelengths, intensity would become
infinite
Actual ­ Intensity reaches a maximum and
then starts to fall again at shorter
wavelengths
The UV catastrophe is used to describe the
failure to predict what happened at shorter
wavelengths.…read more

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THE PHOTO ELECTRIC EFFECT
The emission of electrons from a metal surface
when electromagnetic radiation is incident
on it.
Zinc plate given a negative charge, electrostatic
repulsion of like charges cause leave to repel
and rise.
When UV radiation is incident on surface, leaf falls
showing electroscope is losing its negative
charge ­ electrons are being lost from surface.…read more

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EINSTEINS PHOTOELECTRIC
EQUATION
Electromagnetic energy is quantised. A quantum of
electromagnetic radiation is called a photon.
Photon energy E=hf
For c=f, it follows that E=hc/
If hf< insufficient energy to liberate electron
If hf= electron can just leave surface, f is threshold
frequency
If hf> energy to spare which becomes energy of electron
hf = +Ek(max)…read more

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EXPLAINING THE OBSERVATIONS
IN THE GOLD LEAF
ELECTROSCOPE EXPERIMENT
· Glass plate absorbs high energy photons so
not enough energy to release electrons from
electroscope
· Visible light is below work function of zinc
therefore no electrons released
· Although lots of low energy photons in intense
light, each one is still incapable of liberating
an electron
· Even if very few photons of UV, each one can
liberate electron…read more

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WHY DOES THE WAVE THEORY
FAIL TO EXPLAIN PHOTOELECTRIC
EFFECT?
· According to wave theory, energy arrives at the
surface continually
· Therefore low frequency light of high
intensity should liberate electrons but it does
not
· Weak radiation should lead to an electron taking
a long time to gain sufficient energy to become
free, but provided the frequency is high enough,
emission starts almost instantly…read more

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