Module 7: The Nature Of Light

Flashcards covering the topics in Module 7 lecture notes.

What were the two major breakthroughs of Maxwell's theory of electromagnetism?

James Clerk Maxwell's theory unified electricity and magnetism and predicted electromagnetic waves.

What formula did Maxwell use to predict the velocity of electromagnetic waves?

c = 1 / √(ε0μ0)

Explain the production and propagation of electromagnetic waves according to Maxwell's theory.

A moving charge generates a magnetic field, and a changing magnetic field generates an EMF, leading to propagating electric and magnetic fields.

What is the relationship between the speed of light (c), frequency (f), and wavelength (λ)?

c = fλ

Describe Galileo's experiment to measure the speed of light.

Observer A would uncover his light, and Observer B would uncover his upon seeing A's light, with A measuring the time between uncovering his lamp and seeing B's light.

Explain Roemer's method for determining the speed of light.

Roemer observed variations in the period of revolution of Io, Jupiter's moon, and attributed these to the changing distance between Earth and Jupiter.

How did Fizeau measure the speed of light?

Fizeau used a spinning toothed wheel and a mirror to measure the time it took for light to travel a distance and return.

How do incandescent filaments produce light?

Incandescent lights heat a metal filament to produce electromagnetic radiation, including infrared (heat) and visible light.

How do discharge tubes produce light?

Discharge tubes (e.g., fluorescent lights) pass current through a low-pressure gas, causing it to emit light, often in the ultraviolet range, which then excites a phosphor coating to emit visible light.

How can spectroscopy be used to identify elements?

Each element or molecule has a unique absorption or emission spectrum that can be used to identify it.

Describe the principles of Atomic Absorption Spectroscopy (AAS).

Atomic Absorption Spectroscopy (AAS) is used to identify and quantify small concentrations of metal ions in samples by analyzing the absorption of specific wavelengths of light.

Explain how emission spectroscopy works.

Emission spectroscopy involves vaporizing a sample and analyzing the emitted light spectrum to determine its elemental composition.

How can the spectra of stars provide information on surface temperature?

Peak wavelength is inversely proportional to an object's temperature (Wien's Law).

How can the spectra of stars provide information on translational velocity?

By measuring the Doppler shift of stellar spectra, we can determine if a star is moving towards (blue shift) or away from (red shift) us.

How can the spectra of stars provide information on rotational velocity?

The degree of spectral line broadening due to the Doppler effect reveals the rate of rotation.

How can the spectra of stars provide information on density?

Density = Mass / Volume (volume is derived from radius using the Stefan-Boltzmann Law, and mass is estimated using brightness and temperature).

How can the spectra of stars provide information on chemical composition?

Analysis of spectral bands (emission or absorption) reveals the elements present in a star's atmosphere.

What is Huygens' Principle?

Each point on a wavefront can be considered a source of secondary wavelets, which combine to form a new wavefront.

What is diffraction?

Diffraction is the bending of waves as they pass through an opening or around an obstacle.

What effects the degree of diffraction?

Whether the wavelength is smaller, equal to, or larger than the size of the gap.

What is the extent of diffraction propotional to?

Extent of diffraction is proportional to the ratio of wavelength to width.

Explain Young's Double-Slit Experiment.

Monochromatic light passing through two narrow slits creates an interference pattern of bright and dark bands on a screen.

When does constructive interference occur in Young's Double-Slit Experiment?

Constructive interference occurs when the path difference (pd) is an integer multiple of the wavelength (pd = mλ).

When does destructive interference occur in Young's Double-Slit Experiment?

Destructive interference occurs when the path difference (pd) is a half-integer multiple of the wavelength (pd = (m - 1/2)λ).

What formula is used to express constructive interference?

d sin θ = mλ

What formula is used to express destructive interference?

d sin θ = (m − 1/2)λ

What is polarisation?

Polarization is when a transverse wave is allowed to vibrate in only one direction.

What is Malus's Law?

I = Imax cos²θ

What causes black body radiation?

Objects with an internal temperature above 0 K emit electromagnetic radiation due to the vibration of atoms.

What is a black body spectrum?

The black body spectrum is the continuous spectrum of radiation emitted by a black body.

What does classical waver theory predict regarding black body radiation?

Classical wave theory predicted that as the wavelength of radiation emitted becomes shorter, the intensity would increases.

How did Max Planck respond to the ultraviolet catastrophe?

Max Planck proposed that energy radiated from a blackbody was emitted in discrete packets of energy called quanta.

What is Wien's Law for black body radiation?

λmax = b/T (where b is Wien's displacement constant).

What is Planks' equation?

E = hf

What is an electron volt (eV)?

An electron volt (eV) is the energy gained by an electron moving through a potential difference of 1V.

What is the photoelectric effect?

The photoelectric effect is when electromagnetic radiation incident on a metal plate causes electrons to be ejected.

What is the threshold frequency?

For a particular cathode, there is a certain frequency of light below which no photoelectrons are observed (threshold frequency f0).

What is the relationship between light intensity and photocurrent?

Light intensity increases with photocurrent.

How did Albert Einstein expand on the concept of quanta?

Albert Einstein assumed light exists as photons, each with energy hf.

What is the work function formula?

ϕ = hf0

What formula expresses the relationship between the frequency and kinetic energy of photoelectrons emitted?

Kmax = hf − ϕ

What are Einstein’s two postulates?

All inertial frames of reference are equivalent, and the speed of light in a vacuum is an absolute constant

What is loss of simultaneity?

Loss of simultaneity is when two events which are simultaneous in one inertial frame are not simultaneous in another interial frame.

What is spacetime?

Time and space are interrelated in a four-dimensional relationship called spacetime.

What is the time dilation formula?

t = t0 / √(1 − v²/c²)

What is the length contraction formula?

l = l0 √(1 − v²/c²)

What does the Lorentz factor express?

How much measurements of time, length and other physical properties change for an object white the object is moving

Describe the Hafele-Keating experiment.

Atomic clocks placed on aeroplanes flew around the world and confirmed Einstein's theory of special relativity and time dilation.

How do cosmic-origin muons demonstrate time dilation?

Muons are unstable subatomic particles created in the upper atmosphere that are detected at the Earth's surface due to time dilation.

What is proper time?

Proper time is the time between two events that occur at the same point in space.

What is proper length?

Proper length is the distance between two points whose positions are measured by an observer at rest with respect to the two points.

What is the relativistic momentum formula?

pv = γm0v

What is the limitation on the maximum velocity of a particle imposed by special relativity?

As velocity approaches c, the relativistic mass approaches infinity, making it impossible to accelerate to the speed of light.

What is Einstein's mass-energy equivalence relationship?

E = mc²

What is nuclear fusion?

Nuclear fusion combines light nuclei to form a larger nucleus, releasing energy due to the mass defect.

What is electron-positron annihilation?

A positron collides with an electron at low energies, annihilation occurs and gamma rays are produced.