Physics 3/4 Ch.7-10

What is wavelength, λ?

The distance between two consecutive points in the same phase of a wave.

What is frequency, f?

The number of complete oscillations per second, measured in Hz.

What is period, T?

The time taken for one complete oscillation.

What is the relationship between frequency and period?

f = 1/T

What is the wave equation?

v = fλ

What is a transverse wave?

A wave in which the oscillations are perpendicular to the direction of wave travel.

What is a longitudinal wave?

A wave in which the oscillations are parallel to the direction of wave travel.

What is superposition?

When waves overlap, their individual displacements add to form a resultant displacement.

What is constructive interference?

Interference where waves combine to produce a greater amplitude.

What is destructive interference?

Interference where waves combine to reduce or cancel the amplitude.

What is path difference?

The difference in distance travelled by two waves from their sources to the same point.

What path difference gives constructive interference?

PD = nλ, where n = 0, 1, 2, …

What path difference gives destructive interference?

PD = (n + 1/2)λ, where n = 0, 1, 2, …

What are coherent sources?

Sources that produce waves with the same frequency and a constant phase difference.

What is a node?

A point of zero amplitude caused by destructive interference.

What is an antinode?

A point of maximum amplitude caused by constructive interference.

How is a standing wave formed?

By superposition of a travelling wave and its reflection.

For standing waves required in VCE, what occurs at both ends?

Nodes.

What happens to a wave reflected from a fixed end?

It undergoes a phase reversal.

What happens to a wave reflected from a free end?

It does not undergo a phase reversal.

How is light described in the wave model?

As a transverse electromagnetic wave.

How are electromagnetic waves produced?

Accelerating charges produce changing electric fields and associated changing magnetic fields.

Do electromagnetic waves require a medium?

No.

What is the speed of all electromagnetic waves in a vacuum?

c = 3.00 × 10^8 m s^-1

What equation applies to electromagnetic waves?

c = fλ

How are the electric and magnetic fields oriented in an electromagnetic wave?

They are perpendicular to each other and perpendicular to the direction of travel.

What is diffraction?

The directional spreading of a wave as it passes through a gap or around an obstacle.

What determines the extent of diffraction?

The ratio λ/w, where w is the gap width or obstacle size.

When is diffraction significant?

When wavelength is similar to or greater than the gap or obstacle size; λ/w is approximately 1 or greater.

When is diffraction limited?

When λ/w ≪ 1.

What happens to diffraction if wavelength increases?

Diffraction increases.

What happens to diffraction if gap width decreases?

Diffraction increases.

How does diffraction limit imaging?

Waves cannot clearly resolve details significantly smaller than their wavelength.

What did Young’s double-slit experiment demonstrate?

Light has wave-like properties because it produces an interference pattern.

What is observed in Young’s double-slit experiment?

Alternating bright and dark fringes.

What produces a bright fringe?

Constructive interference.

What produces a dark fringe?

Destructive interference.

Why is the central fringe bright?

The path difference is zero, so the waves interfere constructively.

What is fringe spacing, Δx?

The distance between adjacent bright fringes or adjacent dark fringes.

What is the Young’s double-slit fringe-spacing equation?

Δx = λL/d, when L ≫ d.

What happens to fringe spacing if wavelength increases?

It increases.

What happens to fringe spacing if screen distance L increases?

It increases.

What happens to fringe spacing if slit separation d increases?

It decreases.

What is a photon?

A discrete packet, or quantum, of electromagnetic energy.

What does quantised energy mean?

Energy exists in discrete amounts rather than being transferred continuously.

What is the energy of a photon?

E = hf

What is photon energy in terms of wavelength?

E = hc/λ

What is Planck’s constant?

h = 6.63 × 10^-34 J s

What happens to photon energy when frequency increases?

It increases.

What happens to photon energy when wavelength increases?

It decreases.

What is the photoelectric effect?

The emission of electrons from a metal when electromagnetic radiation of sufficiently high frequency strikes it.

What is the work function, φ?

The minimum energy required to remove an electron from a metal.

What is threshold frequency, f₀?

The minimum frequency required to emit photoelectrons from a particular metal.

How are work function and threshold frequency related?

φ = hf₀

What happens if f < f₀?

No photoelectrons are emitted, regardless of intensity.

State Einstein’s photoelectric equation.

E_k,max = hf − φ

What happens to maximum kinetic energy if frequency increases above threshold?

It increases.

What happens to maximum kinetic energy if intensity increases but frequency stays constant?

It stays the same.

What happens to photocurrent when light intensity increases above threshold frequency?

It increases because more photoelectrons are emitted per second.

In the photon model, what does greater intensity mean?

More photons arriving per unit time, not more energy per photon.

What is stopping potential?

The minimum opposing voltage needed to stop the most energetic emitted photoelectrons from reaching the anode.

How is stopping potential related to maximum kinetic energy?

E_k,max = eV_s

What is the magnitude of the charge of an electron?

e = 1.60 × 10^-19 C

Does increasing intensity affect stopping potential?

No, provided frequency stays constant.

On a graph of E_k,max against frequency, what is the gradient?

Planck’s constant, h.

What is the y-intercept of an E_k,max vs f graph?

−φ, the negative of the work function.

What is the x-intercept of an E_k,max vs f graph?

Threshold frequency, f₀.

Why do different metals give parallel E_k-frequency lines?

All have gradient h, but different work functions.

What photoelectric observation contradicts the classical wave model regarding threshold frequency?

Below a certain frequency, no electrons are emitted regardless of intensity.

What photoelectric observation contradicts the wave model regarding intensity?

Increasing intensity does not increase maximum electron kinetic energy.

What photoelectric observation contradicts the wave model regarding emission time?

Photoelectron emission is effectively instantaneous.

Why does the photoelectric effect support the particle model of light?

It is explained by individual photons transferring discrete amounts of energy to individual electrons.

What did de Broglie propose?

Matter particles can also exhibit wave-like properties.

What is the de Broglie wavelength equation?

λ = h/p

For a non-relativistic particle, what is the de Broglie equation in terms of mass and velocity?

λ = h/(mv)

What happens to de Broglie wavelength as momentum increases?

It decreases.

Why are matter waves most noticeable for very small particles?

Their momentum is small enough for their de Broglie wavelength to be measurable.

What does electron diffraction provide evidence for?

The wave-like nature of matter.

Why can electrons diffract through crystals?

Their de Broglie wavelength can be comparable to atomic spacings in the crystal.

What is the momentum of a photon or matter particle with wavelength λ?

p = h/λ

If a photon and an electron have the same wavelength, how do their momenta compare?

They have the same magnitude of momentum.

What are quantised atomic energy states?

Electrons in atoms can only occupy certain discrete energy states.

What happens when an atom absorbs a photon?

An electron moves to a higher energy state if the photon energy equals the energy difference.

What happens when an electron drops to a lower atomic energy state?

A photon is emitted.

How is photon energy related to the change in atomic energy level?

ΔE = hf = hc/λ

Why do atoms produce discrete emission spectra?

Because electrons can only transition between specific quantised energy levels.

Why do atoms produce absorption lines?

Only photons with energies matching allowed energy-level differences can be absorbed.

How can quantised electron states be explained using matter waves?

Only de Broglie wavelengths that form allowed standing waves are possible, leading to discrete electron states.

What is wave-particle duality?

Light and matter can exhibit both wave-like and particle-like behaviour depending on the experiment.

What provides evidence for the wave nature of light?

Diffraction and interference, including Young’s double-slit experiment.

What provides evidence for the particle nature of light?

The photoelectric effect and quantised photons.

What provides evidence for the wave nature of electrons?

Electron diffraction and interference.

What does the single-photon double-slit experiment show?

Individual photons are detected as particles, but over time they form a wave-like interference pattern.

What does the electron double-slit experiment show?

Electrons exhibit both particle-like detection and wave-like interference.