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A story model for understanding the
A story model for understanding the
photoelectric effect
photoelectric effect
Particles transfer energy in single, rapid events — like two pool balls colliding.
Waves transfer energy continuously.
One wave does almost nothing.
Millions of waves slowly wear a cliff away.
Energy builds up until something happens.
Now imagine something different.
A space alien watches a toy shop.
A boy walks in with 70p.
A ball costs £1.
The shopkeeper refuses.
The boy leaves with no ball — and his 70p still in his pocket.
Another boy tries.
Same thing.
Then another.
Same result.
The alien is confused.
A lot of money has entered the shop.
Why hasn’t it added up to enough to buy a ball?
Then the alien realises something important.
The boys are not like waves.
Their actions don’t add together.
Each visit to the shop is a single event.
Later in the week, the boys return with £1.20 each.
Now every interaction crosses the threshold.
The boys leave with a ball and spare money, throwing the ball into the air with joy as they go.
The meaning is simple:
If there isn’t enough → nothing happens
If there is just enough → the effect happens
If there is more than enough → the effect happens, and the extra becomes motion
No gradual build-up like a wave.
A single event.
Like a particle.
Bringing it back to the photoelectric effect
Bringing it back to the photoelectric effect
Light arrives at the metal in single packets called photons.
Just like the shop:
photon = boy
Each photon brings one amount of energy.
The energy of a photon depends on its frequency:
E = hf
Higher frequency → higher energy
Lower frequency → lower energy
Photon energy (hf) = the money
The energy needed to release an electron from the metal is called the work function:
Φ (phi)
Work function (Φ) = the price of the ball
Rules:
If hf < Φ → no electron is released
If hf = Φ → an electron is released (just escapes)
If hf > Φ → an electron is released and the extra energy becomes motion (kinetic energy)
Extension: from model to physics
Extension: from model to physics
The photoelectric effect is the release of electrons from a metal when light shines on it.
Wave theory suggested that light of any frequency should release electrons, but it might take time for energy to build up.
This does not happen.
What we observe:
Light below a threshold frequency → no electrons
Light above the threshold frequency → electrons are released instantly
This shows that light arrives in packets called photons.
Each photon interacts with one electron.
An electron is only released if the photon energy is high enough:
E = hf
The photon must have more energy than the energy needed to release the electron.
This energy is called the work function.
Increasing the intensity increases the number of photons.
This increases the number of electrons released.
It does not change the threshold frequency.
Exam trap
Exam trap
A brighter light has more photons.
This is like more boys entering the shop.
More photons do not mean more electrons if hf < Φ.
Just like more boys won’t get more balls if they don’t have more money.
Higher frequency means higher energy per photon.
This is like each boy bringing more money.
More photons = higher intensity
More photons ≠ higher energy per photon
hf < Φ → no electrons
When hf ≥ Φ → electrons are released.
Just like when the boys have enough money, balls come out of the shop.
Check your understanding
Check your understanding
What is shone on the metal?
What can be emitted from the metal?
What did wave theory predict?
What is the threshold frequency?
What happens below the threshold frequency?
What happens above the threshold frequency?
What are the particles of light called?
What happens when intensity increases?
Answers
Answers
Light (or UV light)
Electrons
Any frequency would work, given time
Minimum frequency needed to release electrons
No emission
Instant emission
Photons
More photons → more electrons
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