U2: Key Concepts in Physics: Scalars, Vectors, and Motion

Wave

Is a perturbation of some physical quantity that propagates in space and time.

Displacement (of a wave)

Is how far the perturbed physical quantity is from the equilibrium position at a given position and time.

Amplitude (of a wave)

Is the maximum displacement of a wave.

Oscillation (of a wave)

The repeated back and forth movement of a wave.

Period (of a wave)

Is the time it takes for a wave to complete one oscillation.

Frequency (of a wave)

Number of oscillations of a wave that occur per unit of time.

Wavelength (of a wave)

The distance the wave progresses in the periodic time T. Equal to the distance between two matching points e.g. between 2 crests or 2 compressions.

Polarised wave

Transverse waves are confined so that they only oscillate in one plane.

Snell Law

For light travelling from one transparent material to another sin i / sin r is a constant.

Critical Angle

Angle of incidence for which angle of refraction = 90°.

Total internal reflection (TIR)

Reflection of light when travelling from a medium of higher refractive index to lower refractive index and i > c.

Step index optical fibre

Core surrounded by cladding of lower refractive index. Core is very thin to prevent modal dispersion.

Flexible endoscope

A flexible tube for looking inside the body. It has a NON-COHERENT illumination bundle (randomly arranged) and a COHERENT image bundle (parallel arrangement).

Principal focus

The point through which rays of light parallel to the principal axis converge (or appear to diverge from) after passing through a lens.

Focal length

Distance between the optical centre of the lens and the principal focus.

Real image

Image which rays of light pass through and which can be formed on a screen.

Virtual image

Image which rays of light appear to pass through - cannot be formed on a screen.

Healthy range of vision

From 25 cm to ∞.

Myopia/Short sightedness

Inability to see distant objects sharply since image is in front of the retina. Corrected with a diverging lens where f = person's actual far point.

Hypermetropia/Long sightedness

Inability to see close objects clearly since image occurs behind the retina. Near point is beyond the normal 0.25 m, corrected with converging lens.

Principle of superposition

If two waves of the same type travelling in the same medium meet: The resultant displacement is the vector sum of displacements of each wave.

Standing Wave

Formed when two progressive waves travelling in the same medium in opposite directions with the same frequency and amplitude meet and superpose. A stationary wave does not transfer energy; it stores it.

Node

Position of zero displacement.

Antinode

Position of maximum displacement.

Fundamental frequency f0

The lowest frequency a system will naturally vibrate at.

Length of the medium

L = 1/2 λ [string fixed at both ends], L = 1/2 λ [pipe open at both ends], L = 1/4 λ [pipe open at one end]

Coherent waves

Waves of the same type, same frequency/wavelength, maintain a constant phase difference, amplitudes do not have to be equal.

Example of coherent light

Lasers

Laser

Light Amplification by Stimulated Emission of Radiation

Interference

When coherent wave sources meet and constructive or destructive interference occurs forming a pattern of equally spaced bright and dark fringes.

Path difference

The difference in the distances two waves have travelled.

Constructive interference

Path difference = n λ

Destructive interference

Path difference = n + 1/2λ

Resonance

An increase in the amplitude of a vibration that occurs when external vibrations match an object's natural frequency.

Diffraction

Spreading out of waves as they pass through a narrow gap, pass an edge, or go past an object. Maximum diffraction occurs when λ = gap size.

Photon, γ

A packet or quantum of electromagnetic energy.

Photoelectric effect

The emission of electrons from the surface of a metal by absorbing photons.

Electron-volt eV

Unit of energy equivalent to the amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. 1eV = 1.6 × 10−19 J

Photon energy

The energy of a quantum of light (photon) is a defined amount and depends on the frequency and Planck's constant. E = hf or E = hc/λ (h = 6.63 × 10−34 J s)

Work function, Φ

The minimum amount of energy required to just liberate an electron from the surface of a metal and allow it to escape to an infinite distance from the metal.

Energy Levels

An electron has a fixed amount of energy in each orbit; closest to the nucleus = least energy, furthest = most energy. An electron an infinite distance from nucleus has zero energy.

Ground State

When all the electrons in an atom have their lowest possible energy the atom is considered to be in the ground state.

Excitation

An electron absorbs a photon of energy and moves to a higher energy level.

Relaxation

An electron moves from an excited state to a lower level emitting a photon of energy.

Population Inversion

When the number of electrons within a group of atoms exist in greater numbers in an excited state than in the ground state.

Metastable state

Energy level with longer lifetime than normal state.

Stimulated emission

A stimulating photon of energy hf = ∆E causes an electron to relax to the ground state and emit a photon with the same phase, frequency, polarisation and direction.

Wave-particle duality

All matter exhibits both wave and particle properties and can be viewed in one way or another but not both simultaneously.

Wave model

The wave nature of a phenomenon with observable effects such as diffraction, interference and polarisation (as exhibited by E-M waves).

Particle/Photon model

The particle nature of a phenomenon where a particle exhibits momentum or exists as a 'packet' of energy such as a quantum.

Electron Diffraction

If a beam of electrons is passed through a graphite crystal, a diffraction pattern of light and dark concentric rings is observed.

CT Scan

Computed Tomography; uses X rays to produce a slice (cross-sectional) image of the object.

Doppler red shift

The change in wavelength (and frequency) of waves due to the movement of the source relative to the observer.

Cosmological red shift

The change in wavelength (and frequency) of waves due to the expansion of space.

Recession speed

Speed at which distant galaxies are moving away from Earth.