Wave Particle Duality

Quantum Physics

Define wave-particle duality.

The principle that electromagnetic radiation and matter can show both wave-like and particle-like behaviour.

What evidence shows that light behaves as a wave?

Diffraction, interference and polarisation.

What evidence shows that light behaves as particles?

The photoelectric effect.

What evidence shows that electrons behave as waves?

Electron diffraction and interference.

What evidence shows that electrons behave as particles?

Ionisation.

What is Huygens' principle?

Every point on a wavefront can be considered a source of secondary wavelets.

What are secondary wavelets?

Small circular waves produced from points on an existing wavefront.

How does Huygens' principle predict the position of a new wavefront?

The new wavefront is formed from the combined outer edge of the secondary wavelets.

What wave behaviours can Huygens' principle explain?

Reflection, refraction, diffraction, interference and straight-line propagation.

How does Huygens' principle explain diffraction through a gap?

Points across the gap act as sources of secondary wavelets, causing the wave to spread out beyond the gap.

Why is interference evidence that light behaves as a wave?

Interference requires superposition of waves to produce constructive and destructive interference.

Why is diffraction evidence that light behaves as a wave?

Diffraction is the spreading of waves through gaps or around obstacles.

Why is polarisation evidence that light behaves as a wave?

Polarisation is a property of transverse waves.

What happens when light waves constructively interfere?

Their displacements reinforce each other, producing maxima or bright regions.

What happens when light waves destructively interfere?

Their displacements cancel, producing minima or dark regions.

What is a photon?

A discrete packet, or quantum, of electromagnetic radiation energy.

What does the photon model describe light as?

Discrete packets of energy called photons.

What equation gives the energy of a photon?

E = hf.

What does E represent in E = hf?

Photon energy in joules (J).

What does h represent in E = hf?

Planck's constant.

What is the value of Planck's constant?

6.63 × 10⁻³⁴ J s.

What does f represent in E = hf?

Frequency of the electromagnetic radiation in hertz (Hz).

How does photon energy depend on frequency?

Photon energy is directly proportional to frequency.

What happens to photon energy as frequency increases?

Photon energy increases.

What happens to photon energy as wavelength decreases?

Photon energy increases.

What equation relates wave speed, frequency and wavelength for electromagnetic radiation?

c = fλ.

What is the speed of electromagnetic radiation in a vacuum?

3.00 × 10⁸ m s⁻¹.

How can photon energy be calculated from wavelength?

Use f = c/λ and substitute into E = hf, giving E = hc/λ.

What is the equation for photon energy in terms of wavelength?

E = hc/λ.

How does photon energy depend on wavelength?

Photon energy is inversely proportional to wavelength.

Which has more energy: a high-frequency photon or a low-frequency photon?

A high-frequency photon.

Which has more energy: a short-wavelength photon or a long-wavelength photon?

A short-wavelength photon.

Define quantisation.

The principle that certain physical quantities can only exist in discrete values rather than any continuous value.

What does it mean for electromagnetic radiation to be quantised?

Its energy is transferred in discrete packets called photons.

What determines the size of a quantum of electromagnetic radiation energy?

The frequency of the radiation, since E = hf.

What is the minimum step in photon energy for radiation of frequency f?

hf.

Why is Planck's constant important in quantum physics?

It relates the frequency of electromagnetic radiation to the energy of each photon.

How can the frequency of a photon be calculated from its energy?

f = E/h.

How can the wavelength of a photon be calculated if its energy is known?

Use f = E/h and then λ = c/f, or directly λ = hc/E.

Who proposed that light energy could exist as quantised packets called photons?

Albert Einstein.

Which phenomenon provided important evidence for the photon model of light?

The photoelectric effect.

Why could the photoelectric effect not be fully explained using the wave model of light?

Its observations require light energy to be transferred in discrete packets rather than continuously.

What does the photoelectric effect provide evidence for?

The particle nature of electromagnetic radiation.

Why can electrons be considered particles?

They have a fixed charge and can transfer energy and cause ionisation in discrete interactions.

What is the magnitude of the charge on an electron?

1.60 × 10⁻¹⁹ C.

What did Millikan's oil-drop experiment demonstrate?

That electrons have a fixed, quantised electric charge.

Why does ionisation provide evidence that electrons behave as particles?

A fixed amount of charge is removed or added when an atom becomes an ion, showing electrons are localised particles.

How can electrons demonstrate wave behaviour?

They can produce diffraction and interference patterns.

What happens when electrons pass through very small gaps or a double slit?

They can diffract and produce an interference pattern.

Why would classical particle behaviour not predict electron diffraction?

Classical particles would be expected to travel approximately straight through the gaps rather than spread out and interfere.

What does electron diffraction demonstrate?

That electrons can behave as waves.

What does electron interference demonstrate?

That electrons have wave-like properties.

Does wave-particle duality mean light is always simply a wave?

No. Light can display wave-like or particle-like behaviour depending on the experiment.

Does wave-particle duality apply only to light?

No. Matter particles such as electrons can also show wave-like behaviour.

Summarise the wave evidence for light.

Diffraction, interference and polarisation.

Summarise the particle evidence for light.

The photoelectric effect.

Summarise the wave evidence for electrons.

Diffraction and interference.

Summarise the particle evidence for electrons.

Ionisation and their fixed quantised charge.