Physicists John L. Emmett (left) and John H. Nuckolls were the key Lawrence Livermore National Laboratory pioneers in laser and fusion science and technology. Emmett co-invented the multipass laser architecture still in use today. (Lawrence Livermore National Laboratory)

Electromagnetic Theory, Photoelectric effect and Quantum Physics

Quick Review to Date:

Early classical theories of light as a particle predicted that:

light particles have very small masses that vary with colour

-light travels in a straight line

-light had a finite measurable speed

- refraction resulted from light speeding up in a more dense medium (as it does with sound)

Particle theory could not explain:

-diffraction around a barrier

-partial reflection at a boundary

-interference of light (which to be fair wasn't really

something contemporary science could study due to

technological limitations)

Light as Particles

Wave TheoryEarly theories of light as a wave predicted that:

-Light from two in-phase sources would show the same sort of

-interference pattern as water waves with nodes and anti-nodes

-light would slow down in a denser medium

-light could be both reflected and transmitted at the same time

The Electromagnetic Theory

• 1831 Michael Faraday found that he could generate an electrical current by moving a wire in a magnetic field.• There was no theory to explain why this happened.

This was called "Electromagnetic Induction"

Most people didn't get it and didn't see the significance.

Remember that at this time Electromagnetic radiation was not considered to be the same thing as light.

James Clerk Maxwell

Trying to explain Faraday's discoveries....

• 1856 by comparing the behaviour of electricity to water, was able to derive a series of equation that accounted for the electric and magnetic effects in the ether.

• He used the equations to find the speed of these electromagnetic waves and found it to be the same as the known speed of light.

He concluded:

Light consists in transverse undulations(waves) of the same medium which is the cause of electric and magnetic oscillations.

Most people didn't get it and didn't see the significance.

Electromagnetic Theory

• theory predicts that light is an electromagnetic radiation.

• therefore light should have all the properties of electromagnetism and electromagnetism should have all the properties of light.

So now light is not just a wave it is an electromagnetic

wave.

The challenge:

-Find some way to generate and detect electromagnetic radiation.

- 1886 Heinrich Hertz came up with a device to produce EM radiation and to detect it at distances of up to 50 ft.

- He found he could, reflect and refract this radiation (as predicted by Maxwell's theory.

• The main problem was being able to see the spark that the detector produced in the presence of EM radiation.

• He put up a case to shield the detector from visible light and found that the current decreased.

• Found that ultraviolet radiation seemed to be responsible for the increase in current

Huh???

List of Important People who could not Explain the Photo-Electric Effect:

-1888, another German physicist, Wilhelm Hallwachs found out more about the effect on Zinc plates

-1899, J.J. Thomson established that the ultraviolet light caused electrons to be emitted. (Theory is they are shaken loose by light. Give an electron enough light Intensity and it will escape the metal.)

- 1902, Phillip Lenard used a powerful arc lamp to separate the light into different colours. (Found that the intensity of the light was not important but the frequency (or Wavelength) was.

Huh????

http://phet.colorado.edu/en/simulation/photoelectric

Check out the following web site and follow these instructions

http://phet.colorado.edu/en/simulation/photoelectric

-Set Battery voltage to 8.00 volts

-Set Intensity to 97%

-Press play and adjust the slider for the colour of light until you get electrons being ejected-Play around with the different metals and wavelengths of light until you get a feel for what is happening

This is the photo-electric effect which turned out to be a fundamental problem with the wave theory of Light.

1. The electrons were emitted immediately - no time lag!

2. Increasing the intensity of the light increased the number of photoelectrons, but not their maximum kinetic energy!

3. Red light will not cause the ejection of electrons, no matter what the intensity!

4. A weak violet light will eject only a few electrons, but their maximum kinetic energies are greater than those for intense light of longer wavelengths!

The details of the photoelectric effect were in direct contradiction to the expectations of very well developed classical physics.

Photo-electric effect experimenter found:

h=Planck constant (6.626 x 10−34 Js)

Comparing Wave and Particle Explanations for Photoelectric Effect.

Wave Particle

Light as a Wave or a Particle

principle of complimentarity

Albert Einstein

• He proposes that maybe light is a particle and it has to have the right amount of energy to be able to eject an electron from an atom.• Called the Wave-Particle Duality

Principle of Complementarity

• If we measure an object to try to determine where it is it will behave like a particle

• If we do not measure the object it will behave like a wave

• Massive particles (Bigger then a neutron) will behave like particle because we will always know something about its location.

• Smaller ones can be either a wave and/or a particle depending on the situation.

http://www.youtube.com/watch?v=DfPeprQ7oGc

https://www.youtube.com/watch?v=A9tKncAdlHQ

Particle as a wave

Useful Link:http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html

Read pages 49-58

Complete questions 1-5 on pages 59 and 60