Newton and Huygen’s – Learn
Early Theories of Light
Isaac Newton and Christian Huygen’s put forward theories for the Nature of Light in the seventeenth century. Both theories were based on the limited understanding of light at the time and it was difficult for accurate experiments to be conducted to test these theories.
Newton proposed that light consisted of small particles called ‘corpuscles’. His theory meant that, on the smallest scale, a light ray consisted of a shower of tiny particles. Newton published his work in the book ‘Opticks’ and as he was very well respected in the scientific community, his theory was very convincing and widely accepted.
Huygens focussed on the observations that light would create diffraction and interference patterns. He proposed that light consisted of waves, which was able to explain diffraction and interference. Newton’s theory was unable to do this. Due to the lack appropriate equipment, experimental evidence for Huygen’s theory was not available until after his death.
The work and theory of Newton prevailed at the time until the following century when the work of Young and Fresnel provided clear scientific evidence that light, did in fact, behave as a wave as proposed by Christian Huygens.
Diffraction, interference and polarisation, remain as observations that can only be explained and analysed using the theory that light behaves as a wave. Diffraction is the spreading out of a wave after it has passed through a gap or around an obstacle. It can be used to create multiple coherent waves from a single source. Interference occurs when two or more waves act on a point at the same time. It may be constructive, creating a net wave with greater amplitude, or destructive, creating a net wave with smaller amplitude or zero amplitude. Polarisation is a wave behaviour specific to transverse waves; it is the restricting of the plane of oscillation of a transverse wave to just one direction.
Huygens Theory on Light
Huygens’ principle states that each point on a wavefront can be considered as a source of secondary wavelets (i.e. small waves) that travel in the direction of the wave. The new wavefront will be tangential to the wavelets. These wavelets combine to produce a new plane wavefront, as shown below:
Experimental Evidence to Support Huygens
Young’s Double Slit Experiment
In 1803, Young performed an experiment in which he shone monochromatic light on a screen containing two very tiny slits. Monochromatic light is light of a single colour, which means it has a single wavelength and frequency. According to the particle theory, light should have passed directly through the slits to produce two bright lines or bands on the screen. Instead, Young observed a series of bright and dark bands or ‘fringes’ which you would expect from diffracted waves. Young’s double-slit interference experiment provided evidence to support the wave model of light.
Foucault and the Speed of Light
Newton’s theory of light predicted that light would travel faster through water and glass. In the 19th century, Foucault used a rotating mirror to determine the speed of light. He further developed this technique to measure the speed of light as it passed through a 3m tube of water. Whilst the actual speed of light through the water could not be determined, the relative speed of light could be – that is, was it faster or slower. The experiment was based on the deflection of light from the source. The original image reflected off the mirror was offset from the source. If light travelled faster through water, it should deflect toward the source. If it travelled slower through water, it should deflect away from the source. The reflected light deflected away from the source and provided conclusive evidence that light travelled slower through water. As Newton’s theory depended on light travelling faster through water, his ideas were abandoned.