Light Summary
Property
Rectilinear Propagation –
light travels in straight lines
Diffraction – bending
around barriers
Particle Argument
Travels too fast to see it
bend
Imperfect collisions creates
visible fringe effect
Wave Argument
Direction of a wave
Wavelet argument – will
spread if given the
opportunity
Particle supporters argued that light does not
diffract to the extend that waves do
Reflection
Refraction – bending of its
direction when it changes
medium
Both support the law of reflection
Newton, rolling a ball at
The wave theory shows the
different speeds showed
opposite - when θ1> θ2 then
that when θ1> θ2 then the
the particle is moving faster
particle is moving slower in in medium 1
medium 1
This contradiction is important but no one knew
how fast light travelled at this time nor did they
know the speed in various mediums
Partial Reflection Refraction
Dispersion – spreading of
white light into the visible
spectrum
Theory of fits – some fit
some don’t
Each colour has a different
mass – therefore refracts
differently – Red is the
heaviest, Violet the lightest
Easily seen with water
waves (ripple tank)
Each colour has a different
wavelength
Newton’s corpuscular theory was the accepted theory. Instead of defending
his arguments, Newton refuted the wave argument on diffraction (huge
contrast) and the idea of travelling in a vacuum – waves need a medium.
Scientist’s who supported the wave theory had to prove him wrong and in
the 1800’s they did!!!!
Young’s Double slit Experiment (1804) – Showed that light has a
wavelength – Red has the longest, violet the shortest. The colours were
visible due to the interference of light waves – a definite wave property of
light.
Foucault (1850) showed that light travelled slower in water than air. This
solved the contradiction about refraction. Light refracts like a wave.
Polarization effects – light can be reduced to a single vibrational plane with
polarizing filters (crystals) (mid 1800’s) also supporting the wave theory.
Maxwell’s equations (1864) relating Electricity and Magnetism provides a
way that waves can travel in a vacuum but as the video indicated, Maxwell
was not well understood and this fact was not connected at the time.
Aside – Poisson’s Bright Spot – A wave supporter name Augustin Fresnal –
in 1819 devised equations supporting the wave theory of light. Simon
Poisson, a particle supporter argued against the equations stating that if they
were correct, a bright spot should appear at the center of a shadow around a
circular disk. In an attempt to prove Fresnal wrong, a bright spot was
noticed and it became known as Poisson’s Bright Spot.
Further Wave support – colour distortions in soap bubbles can be easily
explained through the interference of waves. Partial reflection refraction
occurs at each medium change and the splitting of the wave creates a path
difference. When these waves experience further medium changes, the
resulting interference creates both constructive and destructive interference –
creating different colours.
To solidify the wave theory of light, scientists had to show how a wave
could travel in a vacuum. This led to the famous Michelson and Morley
experiment (1887) using an interferometer. The conclusion became famous
by Einstein – that the speed of light is ABSOLUTE.