The Photoelectric Effect

1. ThePhotoelectric Effect A level AtThe Sixth Form College Colchester

2. Quantum Physics • 1901 Max Planck initiated “Quantum Physics” with his explanation of Black Body Radiation (Nobel Prize 1918) • Black Body radiation had proved impossible to explain using classical theories of physics

3. A Black Body • Defined as a body that absorbs ALL the radiation that falls on it • All black bodies in the room do reflect – we can see them – they are not Black Bodies • The nearest we can get to a true Black Body is a hole • All radiation that falls on a hole goes through • None is reflected

4. visible ultra violet infra red Black Body Spectrum • Although the Black Body does not reflect energy – it does emit energy • The spectrum of Black Body radiation is a very special shape,

5. Planck’s Explanation • Radiation is emitted (but did not stay) in small packets, he called them quanta (or photons) • The energy E of a packet is given by the equation • E = hf • Where f is the frequency • h is Planck’s constant • h = 6.626 x 10-34 Js

6. Einstein • 1905 Einstein extended Planck’s work by saying that light remains in its photon packets • He used this to explain the photoelectric effect (Nobel Prize 1921 – not won for “Relativity”)

7. The Photoelectric Effect • In the photoelectric effect, electrons are emitted from a metal surface when it is exposed to light • The conditions for this are very particular, and there were problems explaining them with classical physics

8. clean zinc gold leaf Gold Leaf Electroscope • Attached to the top is a zinc plate – which has been cleaned of its oxide layer. • The gold leaf is attached at the bottom, inside the case.

9. Gold Leaf is Repelled • Excess electrons repel each other. • They gather at top and bottom. • The gold leaf is repelled from the metal shaft.

10. ejected electron incoming photon Photo Electrons • Incoming photons fall onto the clean zinc surface. • Electrons are ejected from the surface.

11. Discharged Electroscope • As the electrons leave the surface, the gold leaf is less repelled from the metal shaft. • It falls slowly down.