Wave-Particle Duality (AQA A Level Physics): Flashcards

It predicted only two bright regions on the screen, since corpuscular theory treats light as particles travelling in straight lines through each slit, like paintballs fired through two gaps.

Bright fringes form where the two coherent sources constructively interfere; dark fringes form where they destructively interfere.

The interference pattern could only be explained if light diffracted through the slits like a wave, a phenomenon corpuscular theory could not account for.

The interference pattern on the screen occurred because light diffracted through the thin slits, behaving like a wave.

False.

Although it was definite evidence for wave theory, corpuscular theory was not immediately rejected; further experiments, most importantly Fizeau's measurement of the speed of light in water, were needed before wave theory became accepted.

A wave of oscillating electric and magnetic fields, perpendicular to each other, which propagate each other without needing a medium.

The accelerating charge produces an alternating electric field perpendicular to its motion; this produces a perpendicular alternating magnetic field, which in turn regenerates the electric field, and so on (self-propagation).

Because the alternating electric and magnetic fields self-propagate each other, so no physical medium is needed.

A constant that relates electric field strength to the charged object in free space producing the field.

A constant that relates magnetic flux density to the current in free space that produces the field.

Since μ₀ and ε₀ are both constant, Maxwell's equation shows the speed of electromagnetic waves in a vacuum is also constant.

False.

The electric field's oscillation is perpendicular to the magnetic field's oscillation.

A short air gap between wires, across which a large potential difference produced high-voltage sparks that generated radio waves.

A circular wire with a small break, which produced sparks across the break, and a concave metal sheet with two parallel metal rods at its centre, across which an oscillating potential difference was induced.

He placed a metal screen behind the source and measured a stronger signal with the detector, showing some radio waves had reflected off the screen back towards the detector.

When the detector was rotated 90° perpendicular to the path of the radio waves, sparks stopped being produced, showing the waves' magnetic fields oscillated in only a single plane.

He reflected radio waves off a flat metal sheet to produce a standing wave, then passed a detector across it: strong signals occurred at antinodes and none at nodes. This gave the wavelength, which combined with the transmitter's known frequency using v = fλ gave the speed.

The value he obtained was very close to the speed of electromagnetic waves calculated by Maxwell, showing that radio waves are electromagnetic waves.

When an insulator was placed between the transmitter and detector, there was no difference in the signal detected, showing radio waves could penetrate insulators.

False.

The distance between adjacent antinodes is equal to half the wavelength of the wave.

A rapidly spinning wheel with alternating teeth and gaps, placed in the path of a light beam, which periodically blocked or transmitted the light to create regular pulses.

No light returning from the mirror; the returning light was blocked by the tooth.

The speed of light in water was slower than in air.

Corpuscular theory predicted light would travel faster in a denser medium, but Fizeau found light travelled slower in water than in air.

The total path length of light in Fizeau's experiment was 2*d*, from the source to the mirror and back to the observer.

True.

Scientists used to believe light covered distance instantaneously, though some astronomical observations had already contradicted this before Fizeau measured a finite speed.

A theoretical object that absorbs all radiation incident on it and does not reflect or transmit any radiation.

The wavelength of peak intensity decreases (shifts to shorter wavelengths) as temperature increases.

The disagreement between experimentally measured black-body spectra and the spectra predicted by classical wave theory, which predicted an infinite amount of ultra-violet radiation would be emitted as temperature increased.

Classical theory treated electromagnetic radiation purely as a wave, which predicted an infinite emission of ultra-violet radiation as temperature increased, in stark disagreement with experimental data.

Energy that can only be emitted or absorbed in discrete integer multiples of a fixed packet size, rather than continuously.

where n is an integer and h is Planck's constant (6.63 × 10^-34 J s).

Planck assumed oscillators could only emit energy in integer multiples of packets, or quanta, of energy.

False.

At room temperature, the metal cube emits invisible infrared radiation; it only emits significant visible light, glowing red, orange or white, once heated to around 3000 K.

A photon is a discrete packet (quantum) of electromagnetic energy, of size , which Einstein proposed was massless.

Wave theory predicted photoelectrons would eventually be emitted, because energy is transferred continuously and would build up over time until enough had been transferred to remove electrons.

Each photon transfers all its energy to only one electron. If hf is too small to remove an electron, photons cannot combine their energy, so increasing intensity (the number of photons) has no effect.

Each photon transfers all its energy hf to one electron. Any energy above that needed to remove the electron becomes kinetic energy, so a larger hf leaves more energy for the electron.

According to wave theory, an electromagnetic wave transfers energy continuously, rather than in discrete packets.

False.

Increasing intensity only increases the number of photons, and so the number of photoelectrons emitted. The kinetic energy of each photoelectron depends only on the frequency of the radiation.

Einstein explained the photoelectric effect in 1905, building on Max Planck's work on black-body radiation.

The De Broglie wavelength is the wavelength associated with a moving particle, given by where h is Planck's constant and p is the particle's momentum.

By equating Einstein's mass-energy equation with the photon energy equation , giving , so a photon's momentum is .

where m is the electron's mass and e is its charge.

Its De Broglie wavelength decreases.

De Broglie hypothesised that all particles can behave both like waves and like particles.

False.

Since , doubling V only reduces the wavelength by a factor of , not by half.

The work done on the electron by the electric field equals its kinetic energy: .

Electron diffraction is the spreading out of an electron beam as it passes through small gaps (such as those between atoms in a thin graphite film), producing a circular pattern on a fluorescent screen — direct evidence of electrons' wave-like properties.

The diffraction rings move closer to the centre of the screen. Increasing the potential difference increases electron speed, decreasing the De Broglie wavelength, which decreases the diffraction angle for a given gap width.

A thin film of graphite; the gaps between its carbon atoms were small enough to diffract electrons of the De Broglie wavelength produced.

A shorter wavelength allows the microscope to distinguish two points on an object that are closer together, revealing finer detail.

At this wavelength, an electron beam's resolving power becomes fine enough to distinguish individual atoms, so this voltage sets a target for imaging atomic-scale structure.

Diffraction is a property of waves when passing through a small gap.

False.

The pattern is produced by electrons behaving as waves, diffracting through the gaps between carbon atoms, not by particle scattering.

A TEM is a microscope that focuses a beam of electrons using magnetic lenses; the electrons pass through a thin sample and form an image on a fluorescent screen.

It deflects electrons from the electron gun into a wide beam travelling parallel to the microscope's axis, so the beam is incident uniformly on the sample.

It forms an image of the sample by deflecting the outer electrons of the beam towards the central axis, similar to how a convex lens focuses light.

It spreads out the beam from the objective lens, magnifying the image and directing it onto a fluorescent screen.

Passing through the sample slows the electrons, increasing their De Broglie wavelength and reducing resolving power. Electrons are also slowed by differing amounts, giving them a range of speeds and producing a blurrier image.

Electrons are emitted from the electron gun of a TEM by thermionic emission, then accelerated to high speed by a large potential difference.

False.

Not all electrons are slowed by the sample to the same degree, so they have a range of speeds and are deflected by different amounts in the magnetic lenses, producing a blurrier image.

Quantum tunnelling is the process by which electrons, because of their wave-like nature, cross a narrow barrier (such as a small gap) that they could not cross classically, since their matter-wave amplitude is reduced but not zero on the far side of the barrier.

It moves the probe tip in tiny increments (as small as 0.001 nm) in any direction.

The gap between the tip and the surface decreases, so more electrons tunnel across it, increasing the tunnelling current.

In constant height mode, the tip stays at a fixed height and the tunnelling current varies with the surface, producing the image. In constant current mode, the tip moves up and down to keep the current (and gap size) constant, and this vertical motion produces the image.

Only when the barrier (gap) is weak enough, i.e. the distance between tip and surface is small enough, for the electron's matter-wave amplitude to remain non-zero on the far side.

An STM's probe tip is held a few nanometres above the surface of the sample.

False.

Electrons cross the gap by quantum tunnelling, a wave-like effect not permitted by classical physics; classically the electrons would not have enough energy to cross the barrier.