Light. It is one of most common physical phenomena we come across in our everyday life. Over the years people began to think more about light and were trying to decode its characteristics when it suddenly struck them, what is light made of?
This question would change our perceptions of the natural world forever as it opened the doors to explain new, interesting, phenomena.
Prior to the human advancements in nineteenth century two major theories existed about the nature of light: the wave theory and the particulate theory. On one side we had giants like Christiaan Huygens, Rene Descartes and James Maxwell who advocated that the light behaves as a wave and on the other side we had the expertise of Sir Isaac Newton who advocated for the particulate nature of light.
The stage was now set to determine who was right and why. We will analyse the arguments raised both the sides and finally crown the winner!
Particulate Theory of Light
'' Nature and nature's laws lay hid in the night. God said, Let Newton be! and all was light!''
- Alexander Pope, a famous epitaph intended for Sir Isaac Newton
The Particulate theory of Light was proposed by Pierre Gassendi. Sir Isaac Newton who was deeply influenced by Descartes happened to read Gassendi's Theory and preferred it over Descartes's notion.
Newton began investing much of his time on studying Gassendi's theory, adding scientific inputs to it to bring out his 'Particulate Theory of Light'.
It was in 1675, that Newton proposed that light was in fact made up of particles and that these particles were emitted in all directions from the source. This notion was clearly a scientific revolution as it questioned the correctness of Huygens's Wave Theory of Light. Newton also questioned a fundamental aspect of Wave Theory by stating that speed of light will definitely increase in a denser medium.
Newton went on to publish his famous book on light and colour, a highly acclaimed book titled, 'Opticks' in 1704
Image 1: The front cover of the first edition of Newton's Opticks.
''Light always travels in a straight line, but waves bend around when obstructed by an obstacle. Thus light as a wave sounds absurd.''
- Isaac Newton
Newton was the first to explain polarisation of light using particulate nature of light. Newton could explain reflection by his theory but could not explain refraction and diffraction (can be explained but Newton's theory is not adequate enough), interference and some other associated characteristics of light.
But due to his commanding and powerful hold on physics, people believed that Newton was right until a more systematic approach to wave theory slammed Newton's notion forever.
Wave Theory of Light
In the year 1690, Christiaan Huygens, a great Dutch scientist put forward his wave theory of light.
Huygens's theory was in fact similar to a Descartes Theory published in 1637 which too advocated for wavy nature of light. According to Huygens light emitted in all directions as a wave in a medium.
The wave theory also postulated that the speed of these waves would slow down on entering a denser medium. It also predicted that light waves interfered with each other and this was tested for the first time by Thomas Young in his famous experiments on diffraction of light. Young created a coherent light source and illuminated a plate containing two slits, the light coming out from these slits were observed by him on a screen.
Portrait 3: Thomas Young
The experiment provided a solid proof to the wave nature of light. How?
When the light coming out from the slits were observed on a screen, Thomas Young noticed the interference pattern of light and dark bands as in a 'barcode'. He reasoned that these interference patterns were due to the wave nature of light which predicted that light waves interfere with one another. Through his elegant experiment Young created a major breakthrough in the field of physics by deposing the classical notion of light which stated that light was particulate in nature.
Figure 1: Demonstrative diagram for Young's Double Slit experiment. S₁ and S₂ represents the two slits and 'S' denotes the light source
Experimental and theoretical results of Leonhard Euler and Andre Marié Ampere suggested that Newton's Particulate theory of Light was wrong.
The wave theory could very well explain the phenomena of reflection and refraction of light (using Huygens's Principle), the phenomenon of interference and diffraction of light, etc which could not be explained by Newton's Theory.
So the supporters of wave theory posed all these notion before the public and proclaimed the light behaves as a wave.
Newton's Theory was completely deposed after James Clerk Maxwell described light as a propagating wave in electric and magnetic fields that traveled with a speed equal to 'c'.(which is a constant whose value is equal to 3×10⁸ m/s, the speed of light)
But is it so? The actual truth of the nature of light came after Hertz's Photoelectric Effect and Albert Einstein's Nobel Prize winning explanation to it...
The Photoelectric Effect
Figure 2: A schematic representation of photoelectric effect.
Things changed with in the year 1887, a German physicist Heinrich Rudolf Hertz conducted the famous 'photoelectric effect'.
The experiment was simple but its experimental results literally shook the whole world.
Portrait 4: Heinrich Rudolf Hertz
First let us see how he conducted the experiment.
We know metals contain electrons in them. Hertz allowed light (electromagnetic wave), to hit the metal surface and found that the electrons on the metal surface were ejected at that very instant.
Hertz could also make out that when the intensity of the light was increased the number of electrons ejected from the metal surface also increased. Hertz believed that electrons posses certain threshold frequency and when the frequency of the incident radiation (light) was greater than the threshold value, the electrons were ejected from the metal surface.
Albert Einstein and modern physics
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Portrait 5: Sir Isaac Newton, the Father of Classical Mechanics and a strong supporter of corpuscular theory of light.
In the year 1905, Albert Einstein used Planck's Quantum Theory (1900) to explain the photoelectric effect.
Einstein explained the photoelectric effect using the following postulates,
1) According to Einstein, light is made up of photons and when these photons strike a metal surface it transfers the energy to the electrons and electrons with this energy gets ejected from the metal surface.
2) If the photon possess greater energy, and greater is the kinetic energy possessed by the ejected electrons.
3) Greater the intensity of the incident light greater will be the number of photons in it and greater will the number of electrons ejected from the metal surface.
The results of photoelectric effect questioned the fundamental ideas of wave nature of light.
The wave theory theorised that when intensity increases, the energy possessed by the ejected electrons also increased however in reality it is not the case. Even if you increase the intensity it will not after the kinetic energy of electrons emitted from the metal surface as energy is independent of intensity of incident light.
E=h𝝂 (Einstein-Planck Equation)
E∝𝝂 (where 𝝂 is the frequency of incident light)
It could not explain the instantaneous ejection of electrons from metal surface.
Another reason why wave theory failed is because it could not explain a phenomena called 'black body radiation'.
Conclusion
The most obvious question that you might have in your mind would be, if light is neither of wave or corpuscular origin, then what is it made of?
Einstein explained to us that,
'Light behaves as a wave in some and as a particle in some other cases. But in some unique conditions it behaves both as a wave and as a particle'
So who is the winner?
Newton or Huygens?
We will have to share the prize among the two since both of them were right in the ideas they discovered...
References:
1. NCERT Class XI Chemistry Textbook I
2. NCERT Class XII Physics Textbook Part II
3. All the images published in this blog were taken from different sources available in the internet.
*All the media published in this blog belongs to their original creators and 'Knowledge Through Science' does not claim any right over it.
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