Waves, Photons & Quantum Light – Lecture Review

A comprehensive set of vocabulary flashcards covering the major concepts, laws, experiments, and applications of wave optics, quantum light, and electromagnetic radiation discussed in the lecture notes.

Huygens’ Principle

Every point on a light wavefront acts as a secondary source of spherical wavelets whose envelope forms the next wavefront.

Diffraction

Spreading of waves when they encounter an obstacle or pass through a narrow slit, producing interference patterns of light and dark bands.

Reflection (of light)

Wave property where the angle of incidence equals the angle of reflection measured from the normal.

Refraction

Change in direction of light as it passes from one medium to another due to a change in speed.

Refractive Index (n)

Ratio n = c / v that compares the speed of light in vacuum (c) to its speed in a given medium (v); unit-less.

Snell’s Law

Relationship n₁ sinθ₁ = n₂ sinθ₂ predicting how a light ray bends when entering a new medium.

Critical Angle

Angle of incidence in the denser medium that produces a 90° refracted ray; above it, total internal reflection occurs.

Total Internal Reflection

Complete reflection of light back into a denser medium when incidence exceeds the critical angle.

Dispersion (of light)

Separation of white light into its component colours because different wavelengths refract at different angles.

Chromatic Aberration

Colour fringing in lenses caused by wavelength-dependent refraction; mitigated by achromatic lenses or long focal lengths.

Diffraction Grating

Optical device of many closely spaced slits used to produce and measure detailed diffraction patterns.

Michelson Interferometer

Apparatus that splits and recombines a light beam to create interference fringes for precise distance measurements.

Polarisation

Restriction of a transverse wave’s oscillations to a single plane; proof light is transverse.

Unpolarised Light

Light consisting of waves vibrating in random transverse directions.

Young’s Double-Slit Experiment

Experiment showing light forms an interference pattern, demonstrating its wave nature.

Poisson (Arago) Spot

Bright point in the centre of a circular shadow, confirming wave diffraction predictions.

Blackbody

Ideal object that absorbs all incident radiation and re-emits energy solely according to its temperature.

Ultraviolet Catastrophe

Classical prediction of infinite blackbody energy at short wavelengths, resolved by Planck’s quantisation.

Planck Radiation Formula

Equation giving blackbody spectral intensity, assuming radiation is emitted in discrete quanta E = hf.

Photon

Quantum packet of electromagnetic energy with energy E = hf and momentum p = h/λ.

Photoelectric Effect

Emission of electrons from a metal surface when incident light’s frequency exceeds a threshold, supporting light quanta.

Threshold Frequency

Minimum light frequency needed to eject photoelectrons from a given metal.

Work Function (ϕ)

Minimum energy required to liberate an electron from a metal surface; ϕ = hf_threshold.

Stopping Voltage

Reverse potential that reduces photocurrent to zero, equating to the maximum kinetic energy of photoelectrons.

Saturation Current

Maximum photocurrent when all emitted photoelectrons reach the anode.

Forward Bias (photo-diode)

Condition where cathode is negative, anode positive, aiding photocurrent flow.

Reverse Bias

Condition where anode is negative, repelling photoelectrons and reducing photocurrent.

Quantum (plural Quanta)

Smallest discrete amount of a physical property, such as energy in a photon.

Quantisation

Concept that certain physical quantities take on only discrete values, not continuous ones.

Bohr Model

Atomic model with electrons in fixed orbits of quantised energy around the nucleus.

Ground State

Lowest energy level (n = 1) an electron occupies in an atom.

Excited State

Higher energy orbit an electron occupies after absorbing a photon of exact energy difference.

Principal Quantum Number (n)

Integer labeling electron energy levels; energy Eₙ = –13.6 eV / n² in hydrogen.

Lyman Series

Ultraviolet spectral lines from electron transitions to n = 1 in hydrogen.

Balmer Series

Visible (and some UV) spectral lines from transitions ending at n = 2.

Paschen Series

Infrared spectral lines from transitions ending at n = 3.

Emission Spectrum

Set of discrete wavelengths emitted by excited atoms when electrons drop to lower levels.

Absorption Spectrum

Pattern of dark lines where specific wavelengths are absorbed by a cool gas in front of a continuous source.

Fraunhofer Lines

Dark absorption lines in the solar spectrum caused by elements in the Sun’s atmosphere.

Band Spectrum

Nearly continuous set of lines from molecules having many closely spaced energy levels.

de Broglie Wavelength

λ = h / p; wavelength associated with a moving particle’s momentum.

Electron Microscope

Instrument using electron diffraction to achieve resolutions around 0.1 nm.

Standing Wave (in atom)

Stable matter wave around nucleus where circumference equals an integer multiple of electron wavelength.

Schrödinger Model

Quantum mechanical model describing electron probability clouds (orbitals) rather than fixed paths.

Orbital

Region of space with high probability of finding an electron, derived from Schrödinger’s equation.

Heisenberg Uncertainty Principle

Fundamental limit Δx · Δp ≥ ħ⁄2, restricting simultaneous knowledge of position and momentum.

Wave-Particle Duality

Principle that light and matter exhibit both wave-like and particle-like properties depending on experiment.

Compton Effect

Increase in X-ray wavelength after scattering from electrons, showing photons carry momentum.

Population Inversion

Condition with more atoms in excited state than lower state, essential for laser action.

Laser

Device producing coherent, monochromatic, polarised light via stimulated emission in a gain medium.

Stimulated Emission

Process where an incoming photon triggers an excited atom to emit a second identical photon.

Lasing

Amplification of light in a medium with population inversion, producing a coherent beam.

Semiconductor Laser (Diode)

Laser where electrons cross a band gap in a semiconductor junction, emitting photons used in fibre-optic links.

Synchrotron Light

Intense, highly collimated radiation emitted when relativistic electrons are accelerated in magnetic fields.

Polarising Sunglasses

Eyewear using oriented molecules to absorb horizontally polarised glare while transmitting vertical light.

Photon Momentum

Property p = h/λ enabling light pressure and propulsion methods like solar sails.

Critical Angle Formula

sin θ_c = n₂ / n₁ for light moving from denser (n₁) to rarer (n₂) medium.

Diffraction Pattern

Series of bright and dark fringes from constructive and destructive interference of diffracted waves.

Constructive Interference (Antinode)

Addition of waves in phase, resulting in increased amplitude brightness.

Destructive Interference (Node)

Cancellation of waves out of phase, producing darkness or zero amplitude.

Michelson Path-Difference Formula

ΔL = m λ / 2 for bright fringes in an interferometer (m = integer).

Infrared Radiation

Electromagnetic waves longer than visible light, felt as heat and emitted by all warm objects.

Ultraviolet Radiation

Wavelengths shorter than violet light, capable of inducing fluorescence and skin damage.

X-Rays

High-energy EM waves produced when high-speed electrons decelerate near atomic nuclei; penetrate tissue.

Gamma Rays

EM radiation from nuclear transitions; shortest wavelength and highest energy in the spectrum.

Radio Waves

Longest-wavelength EM waves (≥ 1 mm) used for communication and radio astronomy.

Microwaves

EM waves with wavelengths from 1 mm to 30 cm; used in ovens and wireless communication.

Infrared Thermography

Technique of imaging heat patterns via emitted infrared radiation.

Achromatic Lens

Compound lens combining glasses of different dispersion to minimise chromatic aberration.

Critical Angle (numerical)

Angle satisfying n₁ sin θ_c = n₂ (with sin 90° = 1) for TIR boundary.

Polarising Filter

Sheet of aligned molecules transmitting one plane of polarised light while absorbing orthogonal planes.

Coherence

Fixed phase relationship between waves, necessary for stable interference patterns.

Monochromatic Light

Light of a single (or very narrow band) wavelength.

Spectrum

Range of electromagnetic wavelengths emitted or absorbed by a source.

Spectroscope

Instrument that disperses light into component wavelengths for analysis of spectra.

Kirchhoff’s Laws of Spectroscopy

Rules linking continuous, emission, and absorption spectra to the physical state of a light source.

Fraunhofer Diffraction

Far-field diffraction pattern where wavefronts are effectively planar at the aperture.

Photon Energy

E = hf, where h = 6.63 × 10⁻³⁴ J·s and f is frequency.

Planck’s Constant (h)

Fundamental constant 6.63 × 10⁻³⁴ J·s relating energy to frequency in quantum processes.

Rayleigh-Jeans Law

Classical formula for blackbody radiation that fails at short wavelengths (UV catastrophe).

Band Gap

Energy difference between valence and conduction bands in a semiconductor, determining LED colour.

Photovoltaic Effect

Generation of voltage or current in a material upon exposure to light, basis of solar cells.

Population Inversion

Necessary laser condition where Nexcited ⁄ Nground > 1 in the gain medium.

Fibre-Optic Total Internal Reflection

Guiding of light down a glass fibre via repeated internal reflections due to differing refractive indices.

Planck-Einstein Relation

Photon energy E equals h times frequency f; connects quantum and wave pictures of light.

How do astronomers determine the chemical composition of distant stars?

By analyzing their absorption spectra; dark lines correspond to specific elements in the star's atmosphere that absorb light at unique wavelengths, allowing us to identify their composition.

What causes the iridescent colors seen on CDs or DVDs?

Light diffraction from the microscopic grooves on the disc surface, which act like a diffraction grating, separating white light into its component colors.

How is a star's color related to its surface temperature?

Principles of blackbody radiation show that hotter stars emit more short-wavelength light and appear bluer, while cooler stars emit longer-wavelength light and appear redder.

How do solar cells generate electricity from sunlight?

Through the photoelectric effect, where photons from sunlight strike a semiconductor material, ejecting electrons and creating an electric current.

What is a key application of total internal reflection in communication technology?

Fibre-optic cables (used in internet and telecommunications) guide light signals over long distances by exploiting total internal reflection within the optical fibres.

How are lasers utilized in everyday devices or advanced technologies?

Lasers (Light Amplification by Stimulated Emission of Radiation) produce a focused, monochromatic, and coherent beam of light that can accurately read the patterns in barcodes, or perform precise cuts in manufacturing and surgery.

How do LCD (Liquid Crystal Display) screens work to display images?

They use polarising filters and liquid crystals to control the orientation of light, allowing or blocking specific planes of polarised light to create visible pixels.

What phenomenon explains why an object (like a straw) appears distorted when viewed partially submerged in water?

Refraction, as light bends when passing from water to air (or vice versa) due to the change in speed, causing the apparent displacement or bending of the submerged part.

How do electron microscopes achieve much higher magnification than optical microscopes?

By using electrons instead of light; electrons have a much smaller de Broglie wavelength than visible light, allowing for higher resolution imaging of very small structures.

What principle allows night vision cameras to 'see' in the dark or thermography to detect heat?

They detect infrared radiation emitted by objects, as all warm objects emit infrared waves, allowing visual representation of temperature differences even in low light.

How are X-rays used in medical diagnostics?

X-rays are high-energy electromagnetic waves that can penetrate soft tissues but are absorbed by denser materials like bones, allowing doctors to visualize bone structures and internal organs.

What type of electromagnetic waves are commonly used for broadcasting radio or television signals?

Radio waves, which are the longest-wavelength electromagnetic waves, are used to transmit information over long distances for communication systems like radio and television.

How do passive 3D glasses create the illusion of depth in movies?

They use polarising filters that transmit different planes of polarised light to each eye, mimicking how our eyes perceive depth from two slightly different perspectives.

What optical phenomena are responsible for the formation of a rainbow?

A combination of refraction (light bending as it enters and exits water droplets), reflection (light reflecting inside the droplets), and dispersion (white light splitting into its component colors).

How does the Bohr model explain the characteristic spectral lines of elements?

It posits that electrons exist in quantised energy levels; spectral lines are produced when electrons transition between these levels, emitting or absorbing photons of specific energies (and thus wavelengths).