ELECTROMAGNETIC RADIATION & QUANTUM PHENOMENA

Photon

A packet of light energy.

Photon energy = hf = hc / 𝛌

Photoelectric effect

When light above a threshold frequency is incident on a metal, electrons absorb photons (one to one relationship) and the electrons gain enough energy to be released from the metal.

Threshold frequency (f0)

Minimum frequency of incident electromagnetic

radiation required to release electrons from the surface of a metal

Work function (ɸ)

Minimum energy required by an electron to escape the surface of the metal

photoelectric effect equation

hf = work function + K.E. max

hf = ɸ + ½ m𝑣2

Stopping potential

The minimum negative voltage required to stop

electrons with the maximum kinetic energy.

Equal to K.E. max / e (= K.E. max in eV)

Ground state

The lowest energy state of an atom.

Excited state

When one or more of its electrons moves to a

higher energy level, the atom is in an excited state.

Excitation

An electron jumps up to a higher energy level of an atom when it absorbs a photon of the right

frequency

Ionisation

When there is sufficient energy for an electron to become excited enough to leave the atom.

Ionisation energy is (photon) energy required for an electron to be freed i.e. to reach n=infinity.

De-excitation

An electron goes down one or more energy levels after a certain amount of time and emits a photon

(with the same energy as the difference between the energy levels).

de Broglie wavelength

Wavelike behaviour of a matter particle characterised by a wavelength, λ = h/(mv)

Fluorescent tube

1. High voltage applied across tube, e⁻ flow from cathode to anode, producing e⁻ beam

2. Beam e⁻ collide w/ e⁻ in Hg vapour transferring Ek

3. e⁻ in Hg are excited & move to higher energy level

4. High level = unstable so e⁻ de-excite emitting photons in UV range

5. UV photons collide w/ e⁻ in phosphor coat & get excited to higher energy level

6. Phosphor e⁻ de-excite, emit photons in visible light range

Line spectra

When excited atoms emit light of certain wavelengths which corresponded to different colours

- provides evidence that electrons in atoms can only transition between discrete energy levels

Emission spectra

Electrons transition from a higher energy level to a lower energy level

- each transition corresponds to a different wavelength of light and this corresponds to a line in the spectrum

Absorption spectra

ADD

Evidence of light acting as a particle

Photoelectric effect

Evidence of light acting as a wave

Diffraction & interference of light in Young's Double-Slit experiment

The wave theory of light suggests that "Any frequency of light can give rise to photoelectric emission if the exposure time is long enough"

This is wrong because "Photoelectrons will be released immediately if the frequency is above the threshold for that metal"

The wave theory of light suggests that "The energy absorbed by each electron will increase gradually with each wave"

This is wrong because "Energy is absorbed instantaneously - photoelectrons are either emitted or not emitted after exposure to light"

The wave theory of light suggests that "The kinetic energy of the emitted electrons should increase with radiation intensity"

This is wrong because "If the intensity of the light is increased, more photoelectrons are emitted per second"

Electron diffraction

Concentric rings are observed

- a larger accelerating voltage reduces the diameter of a given ring

- a lower accelerating voltage increases the diameter of the rings