The wave theory of light was unable to explain something

known as the “photoelectric effect”

A simple example of the photoelectric effect is the

protective beam across the bottom of a remote control garage

door opener.

How does it work?

The puzzling part was:

• Not all light caused a flow of electrons off of the metal.

• If light was below a certain frequency, it had no impact.

• If it was above a certain frequency, there was a flow of electricity.

Scientists could not explain why light had to be a minimum

frequency to cause the photoelectric effect.

Enter a scientist by the name of Albert Einstein

Einstein expanded an idea put forth by another physicist, Max

Planck

Planck had proposed that hot objects do not emit

electromagnetic energy continuously, as would be

expected if it was in the form of waves.

Rather, Planck thought objects emit energy in small, specific

amounts called “quanta”.

• A quantum is the minimum amount of energy that can be gained or lost by an atom

• This was the beginning on “quantum mechanics”

Planck proposed this relationship between energy and frequency:

E = hvE is energy in Joules

h is Planck’s constant

v is frequency

In 1905, Einstein put forth the idea that electromagnetic

radiation has a dual wave-particle nature.

In other words, light could be thought of as a stream of particles

He called these particles “photons”.

• A photon is a particle of EMR having zero mass and carrying a quantum of energy

Einstein explained the photoelectric effect by suggesting that EMR is absorbed by matter

only in whole numbers of photons.

If the frequency is too low, the energy of a photon is not enough to

knock electrons loose.

Einstein was awarded the Nobel prize for physics in 1921 for his paper on the photoelectric effect

Let’s observe the hydrogen emission spectrum

Explaining the hydrogen emission spectrum

First, how it is different from the spectrum seen when looking at

sunlight?

Some terminology

• Ground state-

• The lowest energy state of an atom

• Excited state-

• A higher energy state than the ground state

The specific lines/colors seen from the hydrogen spectrum have

particular wavelengths and frequencies.

Once we determine the frequency, we can use E=h to find the energy given off by the

photons.

What does that indicate about the energy levels in hydrogen?

It suggests that the electron only can possess specific energy states.

Now scientists tried to put a model together that would fit this

observation.

The Bohr model was incomplete

• It only worked for hydrogen (one electron)

• It didn’t explain the chemical behavior of atoms

The Bohr model

• The electron can only circle the nucleus in certain fixed orbits.

• The orbit closest to the nucleus is the lowest energy level

• The further the orbit is from the nucleus, the higher the energy level.

Quiz

What is the photoelectric effect?

T or FAll light shone on a piece of metal will cause electrons to be knocked

off

Energy of a photon is related to:a) wavelengthb) frequencyc) speed

When electrons are in their lowest energy state they are said to be in the …

• Ground state