Is light a particle or a wave? The great wave-particle struggle #1

Light in Seventeenth century

In seventeenth century, there were very less facilities for experiments. But the nature was nature, as it is now. Many major behaviors of light was known at that time-

• Light cast shadows. Which shows that light travels in straight line.
• Light is reflected from smooth surfaces. The rules of reflection are- (i) The incident ray, the reflected ray and the normal are in the same plane (ii) The angle of incidence is equal to the angle of reflection.
• When light is travelling from one medium to another medium, it bends unless it falls on the second medium normally. The rules of this phenomenon, called refraction, are: (i) The incident ray, the refracted ray and the normal are in the same plane 
• Light comes in different colours such as red, yellow, green, blue, violet, etc.

All the scientist were trying to understand that why light shows these characteristic behaviors of straight line motion, reflection, refraction, colours, etc

Newton's corpuscle theory

Newton, who was respected by all who had interest in science, came out with what can be called "particle model of light". According to this model when you put on a bulb, a candle or any other source of light, the source emits special kinds of particles-- the particles of light.

Source- https://en.m.wikipedia.org/wiki/Isaac_Newton

These particles were commonly called corpuscles and the description of light given by Newton, corpuscle model of light. The model was very simple in nature and was able to explain the observations available in those periods.

Newton's first law of motion tells that every particle moves in a straight line with a constant speed if no force acts on it. So the particles of light should also move in straight lines in free space‐— a simple explanation for rectilinear motion of light.

Reflection is also simple to understand using the corpuscle model.  A rubber ball hitting a smooth, hard surfaces rebounds. For a perfectly elastic oblique collision from a hard plane surface, the angle of incidence is equal to the angle of reflection, and the two velocity vectors and the normal to the surface are in the same plane. Similarly, a particle of light when strikes a smooth surface surface, say a mirror, reflects obeying the known laws of reflection.

If light goes from air to another medium such as glass, the particles of the second medium attract the particles of light, resulting in deflection in their paths. Once the particle is well within the second medium, force is exerted on it from all sides. This makes the resultant force zero and the particle moves along a straight line. The deflection at the surface causes bending of the light ray. The force of attraction on the particles of light by the medium will also increase their speed and according to the Newton's model, speed of light in water or glass should be larger than that in air.

However, in Newton's times there was no way to measure the speed of light in a medium and this prediction could not be verified. To explain colours, Newton assumed that there are different types of particles of light, each corresponding  to a particular colour.

The corpuscular model of light

Many believe that the corpuscular model of light was known much before Newton. According to Joyce and Joyce, Rene Descartes, in 1637, derived Snell's law using a corpuscular model of light. Newton was only about eight years old when Descartes (1596-1650) died and therefore Descartes did not get the corpuscular model from Newton!

The front covers of the first edition(1704) of OPTICKS: OR, A Treatise of the Reflexions, Refractions, Inflexions and Colours of LIGHT by Newton.

According to Wikipedia,

the corpuscular theory of light, arguably set forward by Pierre Gassendi and Thomas Hobbes states that light is made up of small discrete particles called "corpuscles" (little particles) which travel in a straight line with a finite velocity...... About a half-century after Gassendi, Isaac Newton used existing corpuscularian theories to develop his particle theory of the Physics of light. And it became very famous.

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Huygens wave model

Source- 
https://en.m.wikipedia.org/wiki/Christiaan_Huygens

Newton's theory, though very successful in understanding the known behaviour of light, did not go unchallenged. 

The Dutch physicist  Christian Huygens, a contemporary of Newton, had a very different perspective on this problem. He suggested that light travels as a wave motion in a medium, more like a sound wave. When you put a source of light on, a disturbance is created in the equilibrium state of the medium. This disturbance travels in the medium. But then how does light cast shadow? A sound wave does not appear to cast shadow. If you put a cardboard between yourself and a source of sound, you still 'hear' the sound quite well.

If you keep the same cardboard between yourself and a source of light, you do not see the source. Huygens argued that the wavelength of light waves may be much smaller than the dimensions of the cardboard and the other usual obstacles. In such cases the bending at the edges(diffraction) will be negligible and light will cast a sharp shadow.

Huygens believed that ether vibrated in the same direction as light, and formed a waveitself as it carried the light waves. In a later volume, Huygens' Principle, he ingeniously described how each point on a wave could produce its own wavelets, which then add together to form a wavefront.

The struggle: Newton or Huygens 

The reflection or refraction of light can also be explained on the basis of wave theory. All waves, including sound waves, show these phenomena. Huygens proposed a way to construct the wavefronts and showed that the laws of reflection and refraction are the same for the waves as were known for light. 

However, if light were waves, the speed of light in water or glass should be less than that in air. This was in contrast to the prediction of corpuscle model. But there was no way to measure the speed of light. Hence it was not the deciding factor between wave and corpuscular models. 

Wave theory was having an additional advantage. Different wavelengths of light can be associated with different colours. It was much simpler than Newton's corpuscle model.

At this stage all experimental observations about light could be explained by the corpuscle model and also by the wave model of Huygens. Newton, enjoying a very high respect because of the great success achieved in the field of dynamics, had an advantage and particle model of light by and large remained the popular choice for about 150 years.

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