turning points - wave particle duality

newton’s theory of light

light was formed of tiny particles called corpuscles

huygen’s theory of light

light was a wave and that every point on a wavefront is a point source to secondary wavelets, which spread out to form the next wavefront (Huygen’s principle)

explanations for reflection

newton - corpuscles collide with the surface and a repulsive force pushes them back

this causes the component of velocity perpendicular to the surface to change but the parallel component stays the same

huygens - the whole wavefront will not reach the surface at once (unless it is travelling perpendicular to the surface)

wavelets spread away from the surface once they reach it and rejoin with others to reform the reflected wavefront

explanations for refraction

newton - as the corpuscles approach a denser medium, short-range forces of attraction cause the component of velocity perpendicular to the surface to increase and the parallel component stays the same

light bends towards the normal

light travels faster in denser mediums

huygens - light travels slower in denser mediums

slows down and bends towards the normal

contradiction to newton’s corpuscular theory of light

light travels slower in water

electromagnetic waves

formed of alternating magnetic and electric fields travelling in phase and at right angles to eachother

the direction of wave travel is perpendicular to the oscillations of the fields

permeability of free space

relates the magnetic flux density produced by a wire to the current in the wire

permittivity of free space

relates the electric field strength to the charge on the object, which formed the field.

how did hertz discover radio waves

using apparatus that allowed high voltage sparks to jump across a gap of air

this leads to the production of radio waves

radio waves can be detected by using a dipole receiver

detects the waves’ electric field

made by placing a second set of charged plates parallel to those forming the high voltage sparks

also using a loop of wire with a gap

detects the waves’ alternating magnetic field, as the field will enter the loop

this causes a change in magnetic flux and therefore induces a potential difference, which causes a spark to cross the gap in the wire

how did hertz find the speed of radio waves

place a metal sheet in front of the apparatus

radio waves are reflected back onto themselves

produces stationary waves

calculate wavelength and therefore speed (using the wave equation)

speed found was the same as maxwell’s predicted value of speed of em waves, confirming that radio waves are em waves

how did hertz show that the produced radio waves were polarised

when the receiver is rotated about the line between transmitter and detector the signal varies from max to min after 90 degree rotation

at max the plane of detector is perpendicular to oscillations of e/b field

after rotation of 90, plane of detector is parallel to oscillations of field so no signal detected

how did fizeau measure the speed of light

pulsed beam of light passed through a gap in a toothed wheel rotating slowly

beam reflects on a mirror a large distance behind the wheel, so it returns through the same gap

increase speed of rotation until light cannot be seen

black body

absorbs and emits all possible wavelengths of radiation

ultraviolet catastrophe

wave theory predicts that as the wavelength of radiation decreases, the intensity of the radiation increases, leading to an infinite amount of uv radiation being emitted

not supported by experimental evidence

how was the uv catastrophe resolved

planck’s interpretation of em waves

em waves travel in discrete packets called quanta

how does the photoelectric effect contradict wave theory

wave theory suggests any frequency of light can cause photoelectric emission so doesn’t explain the existence of a threshold frequency

wave theory suggests a time lag but electron emission is instantaneous

wave theory suggests that increasing the intensity increases the speed of photoelectric emission, but it increases the number of photoelectrons emitted per second

photoelectrons are released with a range of kinetic energies

stopping potential

potential difference needed to stop highest energy electrons

electron diffraction

electron gun accelerates electrons through a vaccum tube towards a crystal lattice

electrons interact with small gaps between atoms

diffraction pattern formed on fluorescent screen

resolving power

as the wavelength of the electrons decrease, the resolving power increases

transmission electron microscope (TEM)

electrons accelerated by an electron gun and pass through a set of magnetic lenses and extremely thin sample (so they don’t slow down and wavelength doesn’t change)

purpose of each lens in TEM

condenser lens - deflects electrons so they form a wide parallel beam

objective lens - forms an image of the sample, which is directly above it

projector lens - magnifies image made by objective lens and projects it onto fluorescent screen

how to increase resolving power of a TEM

increase accelerating voltage

wavelength decreases

what limits the resolving power of a TEM

sample thickness - as electrons pass through the sample they slow down, causing their wavelength to increase so resolving power decreases

electrons travel at a range of speeds - some lose ke when leaving metal in electron gun due to collisions, different wavelengths, diffracted by different amounts, causes blurring of image (aberration)

scanning tunneling microscope (STM)

uses quantum tunnelling of electrons to form an image of the surface of an object

how does quantum tunnelling occur

due to the wave nature of electrons

if the barrier they are trying to cross (physical or potential barrier) is small enough, electrons can mov across it

more likely for smaller gaps

how does an STM work

formed of a very fine tipped prove which moves across the surface of an object and stays at a constant potential so electrons can only travel in one direction

movement of electrons is measured and known as tunnelling current

size of gap varies across surface so tunnelling current varies

two ways an STM can operate

constant height mode - probe kept at constant height as it moves across the surface and tunnelling current is used to image the surface of the object

constant current mode - current is constantly monitored and fed back to microscope, allowing it to adjust the probe’s height so current is constant. movement of probe can be used to image the surface