Today’s Learning Outcomes:

Do now:

Read p173-4: Modern Physics & Photoelectric Effect

Wednesday, April 19, 2023

1) What is the photoelectric effect?

2) What were physicists expecting and what did they actually observe?

3) How did Einstein (and Planck) explain this?

Read page 108 and 109

Answer Question 6 and 8

Read page 108 and 109

Answer Question 6 and 8

Read page 108 and 109

Answer Question 6 and 8

Add colour, pictures, diagrams, keywords...But get it done today!

The The photoelectric effect occurs when shining occurs when shining light (usually UV) onto a piece of metal causes light (usually UV) onto a piece of metal causes electrons to be given off.electrons to be given off.

Photo-cells:Thin glass tube (evacuated)

Emitter – curved metal plate

Collector

μA

LIGHT

electron flow

Photoelectric ExperimentsPhotoelectric Experiments

What was expected:What was expected:Brighter light would cause electrons with more Brighter light would cause electrons with more

kinetic energy to be emittedkinetic energy to be emittedWhat actually happened:What actually happened:

Brighter light caused more electrons to be Brighter light caused more electrons to be emitted, but there was no change in the emitted, but there was no change in the amount of energy they hadamount of energy they had

Photoelectric ExperimentsPhotoelectric Experiments

What was expected:What was expected: If very dim light was used, it would take some If very dim light was used, it would take some

time before any electrons had absorbed time before any electrons had absorbed enough energy to escape from the metalenough energy to escape from the metal

What actually happened:What actually happened:With UV light, even the faintest light caused With UV light, even the faintest light caused

some electrons to be emitted instantlysome electrons to be emitted instantly

Photoelectric ExperimentsPhotoelectric Experiments

What was expected:What was expected:The frequency (or colour) of the light used The frequency (or colour) of the light used

would not affect the energy of the emitted would not affect the energy of the emitted electrons.electrons.

What actually happened:What actually happened:The higher the frequency, the higher the The higher the frequency, the higher the

energy of the electrons. Below a certain energy of the electrons. Below a certain frequency, no electrons were emitted.frequency, no electrons were emitted.

Photoelectric ExperimentsPhotoelectric Experiments

Einstein explained these results, using an Einstein explained these results, using an idea suggested by Max Planck, that said idea suggested by Max Planck, that said electromagnetic radiation comes in fixed electromagnetic radiation comes in fixed “packets” or “packets” or quantaquanta of energy called of energy called photonsphotons

The amount of energy each photon has The amount of energy each photon has depends on the frequency of the radiation.depends on the frequency of the radiation.

Photoelectric ExperimentsPhotoelectric Experiments

Each photon has a fixed amount of energy Each photon has a fixed amount of energy given by:given by:

h=Planck’s Constant = 6.63x10h=Planck’s Constant = 6.63x10-34-34JsJsThis suggested that light behaved like a This suggested that light behaved like a

moving particle, rather than a wavemoving particle, rather than a wave

hfE