Physics Light and Atoms

Electromagnetic waves

Made up of mutually perpendicular oscillating electric and magnetic fields

Changing electric and magnetic fields can travel through space from one region to another as

a propagating disturbance called electromagnetic wave

Transverse waves

Electric and magnetic fields oscillate in a plane perpendicular to the direction of the propagation of waves,

Longitude

Electric and magnetic fields oscillate in a plane parallel to the direction of the propagation of waves

Polarisation

The electric field of vectors of an EM wave lie in a plane perpendicular to the direction of wave propagation. The plane containing the electric field vectors is called the plane of polarisation. In a plane polarised EM wave, the oscillating electric or magnetic field is restricted to a single plane or vibration

Path difference

Difference in distance travelled by two waves from their respective sources to a given point on the pattern

Constructive interference

Path difference is m * wavelength. They are in phase

Destructive interference

Path difference is (m+1/2) * wavelength. They are not in phase

Interference pattern

Bright fringes are where waves interfere constructively and dark fringes are where waves interfere destructively

Production of electromagnetic waves

Alternating current flows through the conducting rods of the antenna which produces changing electric and magnetic fields in the space that surrounds the rods. The changing electric and magnetic fields reproduce each other continuously and propagate away from the antenna rids at speed of light.

Sending of electromagnetic waves into antenna

The EM wave is converted back to an alternating current in a receiving antenna. The electric or magnetic field of the incoming EM wave induces a current in the antenna that alternates the same frequency as the incoming EM wave

Monochromatic light

Composed of single frequencies and emits EM waves of visible light in one or many directions

Coherent light

When two or more waves are “in phase”. Emits EM waves with the same frequency, phase, direction and amplitude

Incandescent light

Neither monochromatic or coherent. Emits EM waves with many different frequencies in different directions.

Diffraction of light

Slight bending of light as it passes around the edge of an object or slit. Amount of bending depends on size of wavelength to size of opening

Interference/super position of waves

Where one or more electromagnetic waves overlap. The electric and magnetic fields add vectorially

Transmission diffraction grating

An opaque screen with many closely spaced splits. Each slit acts as a light source when illuminated with light on one side of the grating. dsinθ=mλ

Intensity distribution

The interference pattern has bright fringes which get narrower and brighter as the number of slits increases

Spectroscopy

Science of measuring wavelengths of light emitted by atoms and molecules. Diffraction grating is widely used to disperse light from a sample into a spectrum as it produces sharp and clear fringes to allow for more accurate measurements of the wavelength of light in a spectrum.

Useful properties of grating for spectroscopy

Produces sharp lines of maxima with no overlap or mixture of colours which means an accurate measurement of wavelength of spectral lines. Small slit separation produces a large angular spread which increases accuracy of measurements of wavelengths of a spectral line by providing a better resolution

Wave particle duality

When interacting with matter, light behaves like particles called photons that carry a specific amount of energy which is a multiple of Plancks constant and moves with the speed of light. E = hf

Photon law of conservation of energy

Can be created and destroyed. When a source emits EM waves, photons are created. When photons encounter matter they may be absorbed and transfer their energy to atoms and molecules. Creation and destruction of photons must conserve energy.

Photon momentum

P= h/λ

Photoelectric effect

Proves light consists of massless particles called photons. The emission of electrons from the surface of a conductor when light strikes its surface. The speed and KE of emitted electrons depend on the frequency, not intensity of light

Work function

The minimum amount of energy an electron needs to gain to overcome the potential energy barrier and escape the conductors surface. W=h*f0

Stopping voltage

Reverse potential difference required to stop electron flow completely. Max KE =eVs

Treshold frequency

Minimum frequency needed to knock electrons off a metal surface in the Photoelectric Effect

How are X-rays produced

Produced by accelerating electrons to high speeds and colliding them with a target. Electrons are emitted from a cathode. Both are connected to a high voltage power supply that produces a large PD. The electric field work does work on the electrons which increases its kinetic electron. Electrons collide with the target and some of their KE is transferred into X-rays.

Cathode In X-rays

Metal wire filament heated by the flow of electric current

Why are bones shown in x-rays but not tissues

Absorption of x-ray is perpendicular to atomic number and calcium has a higher atomic number than carbon. Bone is much denser and heavier so it is shown.

Attenuation

Reduction of intensity of an x-ray beam due to absorption of photons as it transverses matter. Depends on the photon energy, thickness, density and atomic number of elements in the material.

Hardness of x-rays

Depends on photon energy and frequency. Hard x-rays have high frequencies. Soft x-rays have low frequencies. Hardness increased by increasing PD between anode and cathode

De broglie wavelength

Wavelength associated with the moving particle. Says that electrons travel through space as a wave λ=h/p

Continuous spectrum

Contains continuous range of wavelengths of EM radiation. When solid, liquid or dense gas are heated to high temperatures they radiate a continuous spectrum which may extend beyond or into the visible region → Incandescence

Line emission spectrum

Shows how electrons in atoms only occupy fixed energy states rather than continuous range of energies. Solids, liquids and high density gases emit a continuous spectrum when heated by low density gases emit a non continuous spectrum when energised by heat or electricity

Atom energy levels

Atoms can be raised to excited states by heating or bombardment with light or particles e.g. electrons. Ground state is the lowest energy. Atom reaches excited state when one or more electrons are in a higher energy level.

Electrons and energy levels

Electrons transition between energy levels by absorbing or emitting a quantum of energy equal to the difference in energy between two energy levels. When transition to lower levels they emit a photon = coloured lines in emission spectrum. Energy of atom decreases

Ionisation of energy of an atom

Minimum energy needed to removed electron from ground state

Why are there no absorption lines in the visible region for hydrogen at room temp

The electron must be in the excited state, n=2 to absorb visible light wavelengths and at room temp the hydrogen electron is at ground state.

Fluorescene

The emission of light by a substance that has absorbed light or other electromagnetic radiation

Process of flurescene

Atom absorbs energy at ground and gets excited but emits two different wavelengths of light on its way back down to the ground state, each with a longer wavelength than the atom absorbed in the visible light range.

Spontaneous emission

Emission of photons that occurs when an electron transitions spontaneously from an excited state to a lower energy level in an atom

Stimulated emission

A photon can stimulate an electron to transition from an excited state to a lower energy level without being absorbed. Occurs when the photon energy is equal to the difference in energy between 2 energy levels. Incident and emitted photons have the same energy, direction and phase

Laser

Device that produce very narrow, intense beam of monochromatic coherent light. The energy carried by a laser beam is concentrated in a small area and travels efficiently over large distances

How do lasers work

Coherent light from stimulated emission is obtained when the atom is excited to a higher energy level so population inversion is produced in which the proportion of atoms in excited states is greater than ground state. The higher energy level must be metastable so the electrons remain longer than usual so the transition to lower level occurs by stimulated emission

Population inversion

A physical state where a system of atoms or molecules has more members in a higher, excited energy level than a lower energy level. Ensures the emission of photons dominate over absorption.

Safety for laser beam

Point away from people before and during operation

Never direct laser at reflective surfaces

Direct beam away from flammable or combustion amterial

Wear protective eyewear specific to laser in use

Cover skin to avoid burns

Quarks

Up, charm, top, down, strange, bottom

Leptons

Electron, muon, tau, electron neutrino, muon neutrino, tau neutrino

Gauge Boson

Gluon, photon, Z boson, W boson

Gauge boson and its fundamental force

Strong nuclear = Gluon

Electromagnetic = Photon

Weak nuclear = W, Z

Gravitational = Graviton

Pion

Composed of a matter-antimatter pair which undergo annihilation.

Baryons and mesons

Baryons are composed of three quarks, and mesons are composed of one quark and one antiquark.

One similarity and one difference between a particle and its antiparticle

Same mass

Opposite electric charge

Describe weak nuclear force in terms of its exchange particles

The weak nuclear force is mediated by the exchange of W and Z bosons during quark and lepton decay. During a decay, the parent particle decays to a daughter particle and a W or Z boson which then decays to two leptons.

Describe the electromagnetic repulsion between electrons in terms of exchange particles

The electrons exchange photons, which causes an impulse directed away from each particle.

Describe strong nuclear force in terms of its exchange particles

The strong nuclear forces of attraction are mediated by the short range exchange of gluons between quarks

Describe electromagnetic force in terms of its exchange particles

The electromagnetic forces of attraction and repulsion are mediated by the short and long range exchange of photons between charged particles

Mesons include

Pion(π), Kaon (K), B, D, eta(η) and rho (ρ)

Baryons include

Proton (P), neutron(n), Lambda(Λ), Delta(Δ), Sigma(Σ), Omega(Ω). Cascade(Ξ)

Baryon number

Of baryon: 1

Anti baryon: -1

Quark: 1/3

Antiquark: -1/3

Lepton number

Lepton: 1

Antilepton: -1

Otherwise 0

Annihalation

When a particle and its antiparticle collide they annihilate releasing energy according to the mass equivalence formula. E=mc²