3(optional) keywords - Turning Points

Source - PMT definitions

Anode

A positively charged electrode

Bertozzi’s Experiment

An experiment that investigated the velocity of electrons emitted with different kinetic energies. It showed that the speed of light was never exceeded and that as the kinetic energy continually increases, the velocity of the electrons continually approaches the speed of light.

Black-Body

A body that emits all wavelengths of radiation, dependent on its temperature

Cathode Rays

Beams of electrons emitted from a cathode through thermionic emission

Cathode

A negatively charged electrode

Condenser Lens

The lens in a TEM that is responsible for deflecting the electrons into a wide beam, that is then incident of the sample

Constant-Current STM

A form of scanning tunnelling microscope in which the current of the probe remains constant, and the probe’s height changes with changes in the sample’s surface. The current that tunnels across varies depending on the height, allowing an image to be formed.

Dipole Detector

A pair of aligned metal rods placed at the centre of a concave reflector

Discharge Tube

A sealed glass tube held at low pressure, with an anode at one end and a cathode at the other. When the electrodes are connected to a supply, the gas in the tube will glow and conduct electricity

Drag Force

A force acting against the oil drop’s motion in Millikan's Oil Drop experiment. All objects travelling through a fluid will experience drag

Electron Gun

A pair electrodes that produce a thin and fast moving beam of electrons. Electrons are emitted from the cathode by thermionic emission and are then accelerated through a gap by the anode

Ether

A substance that was believed to fill all space and be responsible for light propagation

Ether Drift

A theory that the Earth’s ether carried light along with it. This implied that light should travel faster when travelling in the direction of the ether, however this was later proved to be false.

Fizeau’s Determination of the Speed of Light

An early method of determining the speed of light. It involved timing the length of time it took for a beam of light to travel to and back from a mirror, by passing it through a rotating cog wheel

Hertz’s Discovery of Radio Waves

Hertz discovered that radio waves are produced when sparks jump across a gap of air. They can be detected with a dipole detector or a wire loop with a gap in

Huygens’ Wave Theory

A theory stating that all wavefronts consist of a series of points from which secondary wavelets are emitted

Inertial Frame of Reference

A frame of reference which is not accelerating (frames which travel at a constant velocity relative to each other)

Invariance of Speed of Light

The speed of light is identical for all observers, whatever the speed of the source is and whatever the observer's state of motion is

Length Contraction

For an observer moving parallel to a rod, the rod will appear shorter than for an observer at rest relative to the moving rod

Maxwell’s Electromagnetic Theory

Maxwell predicted that a type of wave, known as an electromagnetic wave, consisted of oscillating, perpendicular electric and magnetic fields. He then went onto produce an equation for their speed, which combined with experiments, proved his theory

Michelson-Morley Interferometer

An experimental set-up involving a monochromatic light source, a semi-silvered glass block, a plane glass block, two mirrors and a viewing telescope. It proved that the speed of light is invariant

Negative Glow

A glow found near the cathode in a cathode ray tube. It is caused by the recombination of positive ions with the electrons produced by ionisation. This recombination results in photons being emitted

Newton’s Corpuscular Theory

A theory that light existed as discrete packets, referred to as corpuscles, that travel in straight lines and have momentum. Objective Lens: The second lens in a TEM microscope. It is responsible for magnifying and focussing the image of the sample

Paddle Wheel

A wheel found in a discharge tube that demonstrates the presence of cathode rays

Photon

A packet of energy. Light consists of photons

Projector Lens

The final lens in a TEM that is responsible for focussing the image onto a fluorescent screen

Quantisation of Electric Charge

Electric charge is quantised. This means it can only be found in packets of energy with a charge that is a multiple of the electron charge

Quantum Tunnelling

For a sufficiently thin barrier, the amplitude of a matter wave passing through it will not become zero - this means the matter wave can ‘tunnel’ through the barrier

Relativistic Energy

When an object approaches relativistic speeds, the mass will increase, resulting in an increase of energy known as relativistic energy

Scanning Tunnelling Microscope

A microscope that uses a probe controlled by piezoelectric transducers to scan the surface of a sample. It uses electron tunnelling and has a resolution of around 0.001nm

Specific Charge

The ratio of charge to mass

Stokes’ Law

A law that gives the drag force that acts on a spherical object, such as an oil droplet

Stopping Potential

A value of potential difference at which photoelectric emission is stopped. This is because the electrons no longer have sufficient kinetic energy to cross the potential difference

Thermionic Emission

The emission of electrons from a metal plate due to heating it

Threshold Frequency

The minimum frequency of photons required for photoelectrons to be emitted from the surface of a metal plate through the photoelectric effect. It is equal to the metal’s work function divided by Planck’s constant

Time Dilation

A consequence of the invariance of the speed of light, that means that time will appear to run slowly when you are moving

Transmission Electron Microscope

A microscope that passes a beam of electrons through lenses and across a sample, to form an image on a fluorescent screen. The resolution is around 0.1nm

Ultraviolet Catastrophe

The discrepancy between practical measurements and classical theoretical predictions of the energy intensities at different wavelengths, emitted from a black body