Photoelectricity Quantum Physics Lesson 1

Comment made circa 1900 (Believed to be from Lord Kelvin) "There is nothing new to be discovered in physics now. All that remains is more and more precise measurement"

Learning Objectives Describe the three main conclusions of the photo- electric effect. Define the work function & threshold frequency Describe what the ultra-violet catastrophe was and explain how Planck solved the problem Describe Einstein’s photon model of radiation.  HWK

What is Light? In the late 19 th Century, scientists thought it was a wave and had lots of experimental evidence to back it up. … but there was a problem…

Photoelectric Effect First observed in 1839, by Alexandre Edmond Becquerel. If you shine light on to a metal it can emit electrons.

Safety Don’t look directly into the laser! Put signs on the door. UV only shines on zinc plate.

Photoelectric Effect Try this experiment and observe what happens when you shine a normal light, laser light and UV light on to the negatively charged metal plate. Will the result be the same each time? Make notes of your observations. How can you explain your observations?

Photoelectric Effect What if 100 lasers were directed onto the plate? What if 100 lasers were directed onto the plate? Would that have an effect? Would that have an effect? oror (Diagrams: resourcefulphysics.org)

Important Point to Note One photon releases one photo-electron. Photoelectric PHET simulation:- What happens as the frequency of radiation is changed? What happens as the intensity of radiation is changed?

Wave Theory Predicts… Emission should take place at any frequency. Emission would take place longer using low intensity waves than using high intensity waves.

Three Main Conclusions 1) For a given metal there is a minimum frequency called the threshold frequency below which there is no emission. 2) Photoelectrons are emitted with a range of KE from 0 up to a maximum which increases as the frequency increases. Nothing to do with intensity. 3) Number of photoelectrons emitted per second is proportional to the intensity of the incident radiation.

Why is this important? Only conclusion 3 can be explained if light is a only a wave. Only conclusion 3 can be explained if light is a only a wave. The other 2 conclusions can explained by thinking of light as arriving in discrete packets of energy called quanta. The other 2 conclusions can explained by thinking of light as arriving in discrete packets of energy called quanta. Evidence that light consists of tiny particles called photons! Evidence that light consists of tiny particles called photons!