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AQA A-level Physics: Wave-Particle Duality Cheat Sheet (DRAFT) by

Cheat sheet for turning points optional module for AQA A-level Physics. This cheat sheet only covers the second third of the module, Wave-Particle Duality.

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Newton's Corpuscles

Newton's Corpus­cular Theory of Light

Newton theorised light was made up of particles called corpus­cles.
Reflection caused by corpuscles colliding with surface and repulsive force pushing them back. Caused perpen­dicular component of velocity to change direction, the parallel component stays the same.
Refraction caused by corpuscles approa­ching denser medium. Short-­range forces of attraction causes perpen­dicular component of velocity to increase, parallel component stays the same. Light bends towards normal.
According to Newton's theory, light travels faster in denser mediums.

Huygen's Waves

Huygen's Wave Theory of Light

Huygen theorised light was a wave, every point on a wavefront is a point source to secondary wavelets which spread out to form next wavefr­onts. This is Huygen's principle.
Reflection caused by whole wavefront not reaching surface at once. Wavelets spread away from surface once they reach it and rejoin with others to form reflected wavefront.
Refraction caused by light entering more optically dense medium. This slows down light and makes it bend towards normal.
Newton's theory preferred over Huygen's because Newton had very high reputation at the time, diffra­ction had not yet been observed and the speed of light had not yet been measured.
 

Young's Double Slit Apparatus

Young's Double Slit Experiment

Coherent light shone through two slits so that it diffracts. Each slit is a coherent point source making a pattern of light and dark fringes.
Light fringes formed where light meets in phase and interferes constr­uct­ively, occurs when path difference is a whole number of wavele­ngths.
Dark fringes formed where light meets completely out of phase and interferes destru­cti­vely, occurs when path difference is a whole number plus half wavele­ngths.
If Newton's theory was correct: Only two bright fringes corres­ponding to the two slits. Experiment demons­trated diffra­ction and interf­erence, both properties only explained by Huygen's theory.
Even after experi­ment: Huygen's theory still wasn't accepted because Newton was a historical figure who scientists expected to be correct.
Newton's theory only disreg­arded after speed of light was measured in water, it was shown that light actually moves slower in denser substances contra­dicting the corpus­cular theory.
 

Electr­oma­gnetic Waves

Maxwell's equation for EM waves

Electr­oma­gnetic waves: altern­ating magnetic (B) and electric (E) fields travelling in phase and at right angles to each other. Direction of wave travel perpen­dicular to oscill­ations
Maxwell predicted EM waves existed: Theorised formula for their speed in a vaccum (c) before experiment evidence.
c = 1 / sqrt(μ0ε0)

c = Speed of Light (3x108 ms-1)
μ0 = Permea­bility of free space (4πx10-7 Hm-1)
ε0 = Permit­tivity of free space (8.85x10-12 Fm-1)

Hertz' Apparatus

Hertz' Radio Wave experiment

Hertz discovered radio waves by using apparatus to allow sparks to jump across a gap of air.
Dipole reciever detects electric field. Made by placing second set of charged plates parallel to those forming the voltage sparks.
Loop of wire with gap detects magnetic field as field enters loop causing change in magnetic flux inducing p.d causing spark to cross the gap in the wire.
Metal sheet in front apparatus, radio waves reflect back creating stationary waves. Use one of the detectors to find the distance between adjacent nodes to find the wavele­ngth.
Wavelength multiplied by frequency of the waves to find speed of waves. Hertz's value same as Maxwell's therefore radio waves are EM waves.
Rotating reciever: Plane of detector perpen­dicular to EM waves produced maximum value. Plane of detector parallel to EM waves produces minimum value and no signal detected. Therefore radio waves are polarised.