Origins of Quantum Theory

Planck’s Quantum Hypothesis In the early 1900’s scientists were trying to explain the intensity of the colours emitted when wire is heated. Mathematical equations that worked at one end of the curves did not work at the other.

Max Planck, a German physicist developed an equation the explained the whole curve but only if he hypothesized that the energy of the atoms was NOT continuous but occurred in multiples of a small quantity of energy. Planck was reluctant to pursue this reasoning since it contradicted the classical physics known at the time. Albert Einstein only a few years later proved Planck was correct in his thinking: energy does indeed come in little packets. A packet of energy is known as a QUANTUM of energy

Essentially the quantum view of nature is that, in atoms, there are only certain allowed energies and therefore only definite energy changes possible. (Think of it like books on a book shelf – the books can only be on a shelf; not in between. And to raise a book from a bottom shelf to a higher one takes energy. You explored this concept in Grade 11 when you did The Flame Test Lab. Remember the colour you saw from a particular metal ion was made when the electron was given energy (i.e. heat energy from the Bunsen Burner), causing it to Jump or MAKE A TRANSITION to an EXCITED STATE. Not being Stable it the excited state it returned back to its GROUND STATE and gave off energy in the form of coloured light.)

The Photoelectric Effect In the mid 1800’s, James Maxwell proposed that light is an electromagnetic wave, consisting of a series of wavelengths.

The Photoelectric effect was first discovered by Heinrich Hertz, but he had no explanation for it. It took Einstein to explain it. Photoelectric Effect: The release of electrons from a metal when light strikes its surface. (The removal of electrons leaves the metal positively charged) When the electrons are “kicked off” the metal they have a certain kinetic energy; what was so important is that it was shown it was NOT the intensity of the light that increased this kinetic energy (as classical physics assumed) but rather the frequency (colour/energy) of the light.

Einstein’s Conclusions (to explain the Photoelectric Effect) Light consists of photons – a photon is a quantum of light energy A photon of red light has lower energy than that of UV light The energy of the photon gets transferred to the electron The electron used some of this energy to break free from the atom and some is left over as the electron’s kinetic energy The electron cannot break free if enough energy cannot be absorbed from the photon Different atoms require different amounts of energy for their electrons to be removed

A simply analogy: Einstein, using Planck’s idea of energy being quantized, won a Nobel prize in 1905 for his explanation of the photoelectric effect.