The Photoelectric Effect At The Sixth Form College Colchester A level At The Sixth Form College Colchester

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

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

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

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 The alpha particles scattered in different directions could be observed on the screen. Actually, this was more difficult than it sounds. A single alpha caused a slight fluorescence on the zinc sulphide screen at the end of the microscope. This could only be reliably seen by dark-adapted eyes (after half an hour in complete darkness) and one person could only count the flashes accurately for one minute before needing a break, and count rates above 90 per minute were too fast for reliability. The experiment accumulated data from hundreds of thousands of flashes.

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”) In 1909, an undergraduate, Ernest Marsden, was being trained by Geiger. To quote Rutherford (a lecture he gave much later): "I had observed the scattering of alpha-particles, and Dr. Geiger in my laboratory had examined it in detail. He found, in thin pieces of heavy metal, that the scattering was usually small, of the order of one degree. One day Geiger came to me and said, "Don't you think that young Marsden, whom I am training in radioactive methods, ought to begin a small research?" Now I had thought that, too, so I said, " Why not let him see if any alpha-particles can be scattered through a large angle?" I may tell you in confidence that I did not believe that they would be, since we knew the alpha-particle was a very fast, massive particle with a great deal of energy, and you could show that if the scattering was due to the accumulated effect of a number of small scatterings, the chance of an alpha-particle's being scattered backward was very small. Then I remember two or three days later Geiger coming to me in great excitement and saying "We have been able to get some of the alpha-particles coming backward …" It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

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

Gold Leaf Electroscope clean zinc 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.

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

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

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