Light Summary
Property / Particle Argument / Wave ArgumentRectilinear Propagation – light travels in straight lines / Travels too fast to see it bend / Direction of a wave
Diffraction – bending around barriers / Imperfect collisions creates visible fringe effect / Wavelet argument – will spread if given the opportunity
Particle supporters argued that light does not diffract to the extend that waves do
Reflection / Both support the law of reflection
Refraction – bending of its direction when it changes medium
/ Newton, rolling a ball at different speeds showed that when θ1> θ2 then the particle is moving slower in medium 1 / The wave theory shows the opposite - when θ1> θ2 then the particle is moving faster in 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 / Theory of fits – some fit some don’t / Easily seen with water waves (ripple tank)
Dispersion – spreading of white light into the visible spectrum / Each colour has a different mass – therefore refracts differently – Red is the heaviest, Violet the lightest / 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.