Wave-Particle duality

Newton’s model of light

light was made up of particles called corpucles

Newton’s explanation of reflection

The velocity of the particle parallel to the reflective surface is unchanged and the velocity of the particle perpendicular to the surface is reversed

Newton’s explanation of refraction

Corpuscles are attracted into the surface so their perpendicular velocity increases but parallel velocity is unchanged

Huygen’s theory of light

light is a wave. All points on the wavefront are sources of secondary wavelets.

Huygens explanation of reflection

the whole wavefront doesn’t react the surface at the same time so it reflects at different times and reforms the wavefront

Huygens explanation of refraction

assumed the wave slows down as it enters a more dense medium

what theory for light was preferred?

Newton’s theory was preferred as he had a better reputation

Young’s double slit experiment

coherent light was shone through 2 slits so it diffracts, forming a pattern of light and dark fringes. Light fringes are where the light meets in phase and interferes constructively (path difference is a whole number of wavelengths)

Conculsion from Young’s double slit experiment

Light is a wave as diffraction couldn’t be explained by particle theory - corpuscles would only display 2 fringes, one for each slit

Maxwell’s electromagnetic waves

alternating magnetic and electric fields, travelling in phase at 90^o to each other

what does the electric field strength depend on

permitivity of free space, ε_o

what does the magnetic field strength depend on

the permeability of free space μ_o

how did Hertz discover radio waves?

produced EM waves when a high voltage spark jumped from one metal plate to another. The spark produced a radio wave, which is detected by the dipole by producing a voltage in the detector, causing a spark

what happens when a metal sheet was placed in front of apparatus?

the radio wave is reflected, forming a stationary wave. The wavelength of the stationary wave can be measured by measuring between the nodes and using v=fλ

what did Hertz demonstrate about radiowaves?

• their speed is the same as the speed of light so they are an EM wave

• they are polarised (used a reflector and a dipole) as signal was strongest when parallel

• they cannot be stopped by insulators

how did Fizeau measure the speed of light?

a pulsed beam of light was passed through a gap in a cog rotating at a slow speed. The light reflects off the mirror and returns back through the same gap.

what happens when the speed of the cog increases?

speed is increased until light can no longer pass through. using the frequency of the cog and number of teeth, time can be determine so speed is estimated.

tooth replaces gap every 1/2nf seconds, where n is no. teeth

therefore speed of light is 2d/(1/2nf) = 4dnf

black bodys

absorb and emit all possible wavelengths of radiation

ultraviolet catastrophe

disagreement between the practical and theoretical predictions of the intensity of energy at different wavelengths

what does wave theory predict about the intensity of radiation?

as wavelength decreases, the intensity increases, so predicts an infinite amount of UV radiation

Plank’s resolution to the UV catastrophe

EM waves travel in discrete packets called photons which are not continuous so don’t have infinite energy

photoelectric effect and wave theory