Physics SAT4 - Light and Atoms

EM Waves

Composed of an oscillating E field and B field

Plane of Polarisation

The direction of oscillation, i.e. the E field

Polarised Light

Light with the plane of polarisation on the same plane (same orientation)

Constructive Interference

Occurs when waves are in-phase, PD = m x lambda

Destructive Interference

Occurs when waves are out of phase, PD = (m+1/2) x lambda

Coherence

Two waves maintain a constant phase relationship, must have same wavelength/frequency

Monochromatic Light

Light of a single colour (wavelength/frequency), not necessarily coherent

Incandescent Light

White light produced by a heated substance, not monochromatic or coherent

Diffraction

The bending and spreading of waves when passing an obstacle, slight width approx. equal wavelength

Bright Fringes

When waves are in-phase at the optical screen

Dark Fringes

When waves are out of phase at the optical screen

Creates Coherency

Passing an incoherent light through a single slit

Transmitting Antenna

Attached to AC, causes charge oscillation and transmits EM wave in that plane

Receiving Antenna

Charge oscillates as it receives EM wave in the same plane, interpreted as signal

Conditions for Antennae

Must be in the same plane (both vertical/horizontal) to receive signals

Young’s Double Slit Experiment

Distance between successful maxima (delta y) is assumed to be constant

Intensity Pattern

Drops off from the central maxima, but intensity curve is modulated due to single-slit effects

Transmission Diffraction Grating

Many slits, d = 1/lines per m

Benefits of Diffraction Grating

More precise as d is small, errors have minimal impact. Maxima are thin and intense

White Light Pattern

Central maximum is white, inner maxima are violet as it has small wavelength, further out maxima are red as it has larger wavelength

Photons

Bundles of energy/quanta that behave as a particle

Photon Properties

Zero mass, zero charge, travel at speed of light

Photoelectric Effect

EM radiation incident on metallic surface causes electrons to be released, proof of the particle model of light

Photoelectric Effect Process

Light passes through opening in evacuated tube and hits photosurface, which emits electrons. Some electrons reach collecting plate and attracted to now positively charged photosurface. thus moves through external circuit and current detected.

Einstein’s First Postulate

Monochromatic light consists of discrete quanta, called photons

Einstein’s Second Postulate

Photons are absorbed or emitted on an all-or-nothing basis

Einstein’s Third Postulate

If absorbed by the material, a photon transfers its entire energy to one electron, emitting it instantaneously

Electron Energy Variability

Variation in KE of emitted electrons, as electrons require more energy to emit from lower shells

Work Function

Minimum energy required for the least attracted electron to be emitted

Threshold Frequency

Frequency of photon required for least attracted electron to be emitted

Light Intensity

Greater intensity means more photons and more electron emission, but does not affect electron energy

Stopping Voltage

When charge of collecting plate is negative enough so that only most energetic electrons make it across - as V increases, charge becomes more negative

X-Rays

Electrons accelerated across large potential difference to strike a metal target, resulting in high energy photons being released

X-Ray Process

Electrons released by filament as thermal energy is provided, electrons accelerate across V and gain KE, electrons interact with target metal and release photons in x-ray spectrum

High Attenuation

Material scatters more x-rays, caused by thicker/denser material and greater effective atomic number

Low Attenuation

Material absorbs more x-rays

X-Ray Image Creation

Photographic film is white, turns black when protons hit (i.e. dense areas stay white)

Optimising X-Rays

Greater hardness (energy), decreased exposure time

de Broglie Wavelength

Wavelength of moving particles dependent on momentum

Davisson Germer Experiment

Electrons accelerated to 54eV, scattered electrons produce intensity graph that varied with angle

Continuous Spectrum

Continuous range of frequencies/wavelengths/energies of light produced through incandescence, distribution dependent on temperature

Incandescence

All particles in constant random motion, thus oscillating, which produces all frequencies of EM radiation and hence continuous random spectrum

Increased Average KE

Increased temperature, oscillating at higher frequencies

Emission Spectrum

Specific lines produced through exciting electrons, black with coloured bands

Absorption Spectrum

Continuous spectrum with black bands of emitted light, typically corresponding to emission spectrum

Ground State

n=1, electrons requires most energy to emit

Ionisation Energy

Energy requires to remove an electron from the ground state

Photon Absorption

When photon energy corresponds exactly to energy difference, electron jumps to matching energy level

Photon Emission

When excited, electrons will eventually drop to ground state and release photon with energy corresponding to E-level transition

Multiple Photon Emission

Excited electrons make multiple jumps down and release multiple electrons

Lyman Series

Electron drops to ground state, emits UV photon

Balmer Series

Electron drops to n=2, emits visible light

Paschen Series

Electron drops to n=3, emits infrared photon

Production of Absorption Lines

Hydrogen must be heated to excite electron, sit in n=2 instead so visible light is emitted

Fraunhofer Lines

Core of sun produces white light through incandescence, outer layers have electrons existing in energy levels thus the Sun produces absorption spectrum

Fluorescence

Process of converting single high energy proton into multiple smaller energy photos through absorption/emission

Fluorescence Process

High energy photon absorbed, jumps to high energy state, jumps to ground state and emits series of lower energy photons

Stimulated Emission

If electron in metastable state and photon matching energy transition is incident on atom, forced transition occurs. Incident photon not absorbed, and emitted photon is identical

Benefits of Stimulated Emission

Coherent, uni-directional and monochromatic light, only occurs when there is population inversion (majority of electrons in metastable state)

Lasers

Use stimulated emission to produce EM radiation, concentrated in a small area and travel over large distances

Population Inversion

More electrons are in a metastable state than not

Production of Coherent Light

Atoms must be in population inversion, higher E level must be metastable, and mirrors are used to trap emitted photons in system

Fermions

Particles that have mass

Gauge Bosons

Particles that are force carriers

Antiparticles

Identical to their particle counterparts with opposite quantum numbers

Quarks

Fundamental particles that make up hadrons

Leptons

Fundamental particles with six flavours (electron, muon, tau) with neutrino counterparts

Baryons

Hadrons that are made up of 3 quarks, B=1

Mesons

Hadrons that are made up of 1 quark and 1 anti-quark (B=0)

Quark Baryon Number

1/3

Quark Confinement

Quarks cannot be observed in isolation

Lepton Family Number

Have +1 corresponding to their type, and 0 for all other families

Gravity

Fundamental force that acts between anything with mass, weakest

Electromagnetism

Fundamental force that acts between charged particles, relatively strong

Strong Nuclear Force

Fundamental force that acts between quarks/hadrons, strongest but very short range

Weak Nuclear Force

Fundamental force that acts between leptons during beta decay, weak

Photons

Gauge boson particle for electromagnetism

EM Force through Bosons

Virtual photons are instantaneously emitted by charge, causing recoil as photons have momentum. If repel, shoot toward like charge. If attract, shoot toward opposite charge.

Gluons

Gauge boson particle for strong nuclear force

W+, W- and Z0

Gauge boson particles for weak nuclear force

Graviton

Theoretical gauge boson for gravity, not yet observed

Active Medium

Materials in a laser used to emit the light, can be solid, liquid or gas

Pump/Energy Source

Device in a laser that supplies energy to the active medium to excite electrons and lead to a population inversion

Optical Cavity

Structure in a laser that encases active medium and consists of parallel mirrors, which reflect photons back to stimulate photon emission AND allow light to escape as laser beam

Parts of a Laser

Active medium, pump/energy source, optical cavity