1. Light is a Particle Physics 12 Adv

2. Blackbody Radiation A blackbody is a perfect emitter; that is it emits the complete EM spectrum Work done by Gustav Kirchhoff shows that the power radiated by a blackbody depends on the body’s temperature Further, he determined there was a complex relationship between the frequency and intensity

3. Blackbody Radiation According to EM theory, the curve observed in blackbody radiation cannot be predicted It was possible to model the lower frequencies but not the higher

4. Quantum Theory Max Planck was able to determine an empirical mathematical relationship between the intensity and frequency of blackbody data In order to develop a theory that described his relationship, Planck was required to use discrete mathematics

5. Quantum Theory This lead to the idea that there was a minimum amount of energy that could be exchanged This minimum amount of energy lead to the idea that energy was quantized meaning that it can only exist is specific “packets” E = nhf where n = 0, 1, 2, 3, …

6. Quantum Theory Using Boltzmann’s statistical models Planck was able to model the behaviour of blackbody radiation exactly This was published in 1900 and was the birth of modern physics However, these results were not immediately accepted (even by Planck) as they were contrary to previous work

7. Photoelectric Effect The photoelectric effect eventually provided support for the idea of quantization of energy The photoelectric effect occurs when photoelectrons are emitted from a metal when exposed to certain frequencies of light

8. Photoelectric Effect Experiments with the photoelectric effect led to two key conclusions: When the intensity of light increases, the number of electrons emitted increases The maximum kinetic energy of the electron ejected from the metal is determined only by the frequency of light and is not affected by intensity

9. Einstein and the Photoelectric Effect Einstein saw the link between Planck’s quantization of energy and the photoelectric effect He proposed that not only would light be emitted as quanta but must also be absorbed as quanta By considering these quanta (or photons) he was able to explain the photoelectric effect

10. Einstein and the Photoelectric Effect By using the concept of the photon (and its associated energy), Einstein proposed the following: hf = W + E k(max) Despite the fact this equation worked, it was not widely accepted (even by Planck)

11. Millikan and the Photoelectric Effect Because the charge on the electron was not known when Einstein published his paper on the photoelectric effect, his result could not be proven Millikan, having determined the charge on the electron, improved on the photoelectric effect experimental design and was able to confirm Einstein’s assumptions

12. Millikan and the Photoelectric Effect Using his experimental design, Millikan was able to produce data that supported Einstein’s equation: E k(max) = hf – W E k – kinetic energy of photoelectron h – Planck’s constant f – frequency of EM radiation W – work function of metal

13. Practice Problems Page 852 1-4