EM Radiation and Quantum Phenomena - AQA A-Level Physics

Energy level

The energy of an electron in an electron shell of an atom.

Ionisation

Electrons are completely removed from an atom or an atom gain an electron/s, due to ionising radiation or incident particles.

Excitation

An atom’s electron absorbs specific amounts of energy from colliding electrons (or other particles), and moves up to a higher energy level.

De-excitation

An atom loses energy by photon emission as a result of an electron moving to a lower energy level.

Photoelectric effect

The emission of electrons from a metal surface when electromagnetic radiation above a certain frequency is incident on the surface.

Threshold frequency

The minimum frequency of incident electromagnetic radiation required for the photoelectric effect to occur.

Work function

Minimum amount of energy needed by an electron to escape from a metal surface.

Stopping potential

The minimum potential applied to the surface needed to stop photoelectric emission.

Fluorescence

A glow of light from a substance exposed to UV radiation. The atoms de-excite in stages and emit visible photons in the process.

Ground state

The lowest energy level of an atom.

Photon

A packet of electromagnetic radiation

Spectrometer

An instrument used to correctly measure light wavelengths

Explain the 3 steps of how a fluorescent tube works (part 1)

Electrons flow between the electrodes in the tube and collide with mercury atoms, causing them to excite

Explain the 3 steps of how a fluorescent tube works (part 2)

Mercury atoms release UV photons when they de-excite, which excite the atoms in the fluorescent coating

Explain the 3 steps of how a fluorescent tube works (part 3)

The atoms in the fluorescent coating release photons in the visible spectrum when they de-excite

Why is the vapour low pressure in a fluorescent tube?

So the vapour isn’t too dense to absorb the electrons and stop the current.

What will a line emission spectra show?

Bright lines on a dark background

How is a line emission spectra produced?

By passing the certain wavelengths emitted from a hot gas through a prism or a spectrometer

What will line absorption spectra show?

A continuous spectrum with certain wavelengths missing

How is a line absorption spectra produced?

By passing white light through a hot gas, so specific wavelengths of light that match the energy difference between electron shells are absorbed, leaving dark lines in the continuous spectrum.

Give an example of how light shows wave-like nature

Through diffraction and interference patterns caused by superposition

Give an example of how light shows particle-like nature

The photoelectric effect

Electron volt

Amount of energy equal to 1.6×10^-19J, defined as the energy gained by an electron when it is accelerated through a pd of 1V

What is the classical theory for the photoelectric effect?

Incident waves deposit energy given to all the electrons exposed to it until they have enough energy to be emitted.

What disproves the classic theory for the photoelectric effect? (2)

• The effect requires the electromagnetic radiation to have a threshold frequency

• The effect is instantaneous

• Electron energy is dependent on frequency, not intensity

Why is the photoelectric effect not observed bellow the threshold frequency?

Because the energy of each photon is dependent on the frequency, and the energy must be sufficient to meet the metal’s work function.

Give an example of electrons showing particle behaviour

Being deflected in a magnetic field

Give an example of electrons showing wave behaviour

Electron diffraction, which occurs when electrons from an electron gun are passed through a thin graphite screen, creating diffraction rings on a screen.

How come increasing potential difference causes the pattern to squeeze together in electron diffraction?

Increasing potential difference increases the speed and momentum of electrons, and since wavelength is inversely proportional to momentum, the wavelength reduces and there is less diffraction.

What is the effect on energy of electrons when the accelerating pd is increased?

The electrons will have more energy

What is the effect on electrons, with not enough energy to cause excitation, accelerated through a gas?

Electrons will be deflected off the atoms and there won’t be much loss in KE

What is the effect on electrons, with exactly enough energy to cause excitation, accelerated through a gas?

Electrons will lose all their KE and current will be reduced

What is the effect on electrons, with more than enough energy to cause excitation, accelerated through a gas?

There will be no effect on current and atoms will be excited

What formula relating to the photoelectric effect can the straight line equation be applied to?

E_kmax = hf - Φ

How does intensity increase the photoelectric effect past threshold frequency

It increases the nunber of photoelectrons emitted, as more incident photons results in more electrons absorbing enough energy to escape the metal surface.