Turning points

What is the cathode ray?

The glow observed at the cathode of a discharge tube when a potential difference is applied across it.

How does a discharge tube glow?

A high potential difference causes electrons to be pulled off gas atoms, ions accelerate towards the cathode, releasing more electrons, which collide with gas atoms, exciting them and resulting in the emission of photons of light.

What is thermionic emission?

The emission of electrons from the surface of a metal when it is heated.

How does an electron gun operate?

It uses thermionic emission to release electrons from a heated cathode, which are then accelerated towards an anode by a potential difference.

What is the specific charge of an electron?

The ratio of the charge of an electron to its mass, approximately 1.76 x 10^11 C/kg.

Why was Thomson's measurement of the specific charge significant?

It demonstrated that the specific charge was constant regardless of the gas used to produce the electrons, implying that all atoms contain electrons.

How did Millikan determine the charge of the electron?

He used the oil drop experiment to balance the weight of a charged droplet with an electric force, allowing him to calculate its charge.

What did Millikan's results show?

The charge of all oil droplets was an integer multiple of 1.60 x 10^-19 C, indicating that charge is quantised.

What was Newton's corpuscular theory of light?

Light is made up of tiny particles called corpuscles.

How did Newton describe reflection and refraction with his corpuscular theory?

Reflection occurs when corpuscles collide with a surface, becoming repelled; refraction occurs when corpuscles are attracted to a denser medium, causing light to bend towards the normal.

What is Huygens' wave theory of light?

Light is a wave made up of secondary wavelets that propagate from every point on a wavefront.

What was the significance of Young's double slit experiment?

It demonstrated diffraction and interference of light, supporting Huygens' wave theory and contradicting Newton's corpuscular theory.

Why wasn't Huygens' wave theory widely accepted initially?

Newton's reputation and the lack of measurements of light speed in different mediums.

What are electromagnetic waves?

Transverse waves formed of alternating electric and magnetic fields traveling in phase and at right angles to each other.

What is the formula for the speed of electromagnetic waves in a vacuum?

c = 1/√(μ0ε0), where μ0 is the permeability of free space and ε0 is the permittivity of free space.

How did Hertz discover radio waves?

By generating high voltage sparks across a gap of air which produced and could be detected as radio waves.

How did Hertz confirm that radio waves were EM waves?

By measuring their speed using stationary waves, he found it matched Maxwell's predicted value for the speed of light.

How did Fizeau measure the speed of light?

Using a rotating toothed wheel to create light pulses that traveled to a distant mirror and back, allowing calculation of light's speed.

What was the significance of Fizeau's result?

It was close to the value predicted by Maxwell, supporting the idea that light is an electromagnetic wave.

What was the ultraviolet catastrophe?

A prediction that the intensity of radiation emitted by a black body would increase infinitely as wavelength decreased, contradicting observations.

How did Planck resolve the ultraviolet catastrophe?

He proposed that EM waves travel in discrete packets called quanta, with energy directly proportional to their frequency (E = hf).

What aspects of the photoelectric effect could wave theory not explain?

1. Existence of a threshold frequency. 2. Immediacy of photoelectron emission. 3. Independence of photoelectron speed on light intensity. 4. Range of kinetic energies of photoelectrons.

How did Einstein explain the photoelectric effect?

He proposed that EM waves are emitted as discrete packets called photons, transferring energy completely to electrons upon interaction.

What is stopping potential?

The potential difference required to stop photoelectrons with maximum kinetic energy from being emitted.

How can a graph of stopping potential against frequency confirm Einstein's photon theory?

It produces a straight line with gradient h/e, a y-intercept of -Φ/e, and an x-intercept equal to the threshold frequency.

What is the de Broglie hypothesis?

All particles exhibit both wave-like and particle-like behavior; the wavelength is given by λ = h/mv.

What experimental evidence supports the de Broglie hypothesis?

Electron diffraction, demonstrating wave-like behavior as electrons diffract through a crystal lattice.

How does accelerating voltage affect electron diffraction?

Increasing voltage decreases the de Broglie wavelength and fringe spacing; decreasing voltage does the opposite.

What is the resolving power of a microscope?

Its ability to distinguish between closely spaced structures.

Why do electron microscopes have a higher resolving power than light microscopes?

The wavelength of an electron beam is much smaller than that of light.

What are the two types of electron microscopes?

Transmission electron microscope (TEM) and scanning tunneling microscope (STM).

How does a TEM work?

It focuses a beam of electrons using an electron gun and magnetic lenses to pass through a thin sample, creating a magnified image on a screen.

What factors limit the resolving power of a TEM?

Sample thickness and the range of speeds of electrons.

How does an STM work?

It images surfaces by utilizing quantum tunneling of electrons between a fine-tipped probe and the object's surface.

What are the two modes of operation for an STM?

Constant height mode and constant current mode.

What was the Michelson-Morley experiment designed to measure?

The absolute speed of the Earth through the ether.

What did the Michelson-Morley experiment show?

No shift in the interference pattern, contradicting the ether theory.

What are the postulates of Einstein's theory of special relativity?

1. The speed of light in free space is invariant. 2. The laws of physics have the same form in all inertial frames of reference.

What is time dilation?

Time runs at different speeds depending on the relative motion of the observer.

What is proper time?

The time measured by an observer stationary relative to the event being measured.

What experimental evidence supports time dilation?

Muon decay demonstrates slower decay due to time dilation at high speeds.

What is length contraction?

An object's length appears shorter to an external observer when moving at high speed.

What is proper length?

The length of an object as measured by an observer at rest relative to the object.

How does muon decay provide evidence for length contraction?

Muons traveling at high speeds appear to cover shorter distances before decaying, affecting their decay rates.

What is the relationship between mass and energy according to special relativity?

E = mc^2, indicating mass and energy are interchangeable.

What is relativistic mass?

The mass of an object increases with its speed.

Why does the classical formula for kinetic energy not apply at relativistic speeds?

The object's mass changes significantly.

How did Bertozzi's experiment provide evidence for the increase in mass with speed?

By measuring the kinetic energy of electrons at various speeds, his results matched Einstein's predictions.

Can an object reach the speed of light according to special relativity?

No, as speed approaches the speed of light, mass and energy approach infinity.