In optics, the corpuscular theory of light, arguably set forward by Descartes in 1637, states that light is made up of small discrete particles called "corpuscles" (little particles) which travel in a straight line with a finite velocity and possess impetus. This was based on an alternate description of atomism of the time period.
Isaac Newton was a pioneer of this theory, notably elaborated upon by him in 1672. This early conception of the particle theory of light was an early forerunner to the modern understanding of the photon. This theory cannot explain refraction, diffraction and interference, which require an understanding of the wave theory of light.
In the early 17th century, natural philosophers were seeking new information to replace Aristotelianism that had been popular for centuries. Various European philosophers adopted what came to be known as mechanical philosophy sometime between around 1610 to 1650, which described the universe and its contents as a kind of large-scale mechanism, a philosophy that explained the universe is made with matter and motion. This mechanical philosophy was based on Epicureanism, and the work of Leucippus and his pupil Democritus and their atomism, in which everything in the universe, including a person's body, mind, soul and even thoughts, was made of atoms; very small particles of moving matter. During the early part of the 17th century, the atomistic portion of mechanical philosophy was largely developed by Gassendi, René Descartes and other atomists.
Pierre Gassendi's atomist matter theory
The core of Pierre Gassendi's philosophy is his atomist matter theory. In his great work, Syntagma Philosophicum, ("Philosophical Treatise"), published posthumously in 1658, Gassendi tried to explain aspects of matter and natural phenomena of the world in terms of atoms and the void. He took Epicurean atomism and modified it to be compatible with Christian theology, by suggesting several key changes to it:
- God exists
- God created a finite number of indivisible and moving atoms
- God has a continuing divine relationship to creation (of matter)
- Humans have free will
- The human soul exists
Gassendi thought that atoms move in an empty space, classically known as the void, which contradicts the Aristotelian view that the universe is fully made of matter. Gassendi also suggests that information gathered by the human senses has a material form, especially in the case of vision.
Corpuscular theories, or corpuscularianism, are similar to the theories of atomism, except that in atomism the atoms were supposed to be indivisible, whereas corpuscles could in principle be divided. Corpuscles are single, infinitesimally small, particles which have shape, size, color, and other physical properties which alter their functions and effects in phenomena in the mechanical and biological sciences. This later led to the modern idea that compounds have secondary properties different from the elements of those compounds. Gassendi asserts that corpuscles are particles that carry other substance or substances and are of different types. These corpuscles are also emissions from various sources such as solar entities, animals or plants. Robert Boyle was a strong proponent of corpuscularianism and used the theory to exemplify the differences between a vacuum and a plenum, by which he aimed to further support his mechanical philosophy and overall atomist theory. About a half-century after Gassendi, Isaac Newton used existing corpuscular theories to develop his particle theory of the physics of light.
Isaac Newton argued that the geometric nature of reflection and refraction of light could only be explained if light was made of particles, referred to as corpuscles, because waves do not tend to travel in straight lines. Newton sought to disprove Christiaan Huygens' theory that light was made of waves. In his 44th trial in a series of experiments concerning physics of light, he concluded that light is made of particles and not waves by having passed a beam of white light through two prisms which were held at such an angle that the light split into a spectrum after passing through the first prism and then was recomposed, back into white light, by the second prism.
The corpuscular theory was largely developed by Isaac Newton. Newton's theory was predominant for more than 100 years and took precedence over Huygens' wave theory of light, partly because of Newton's great prestige. When the corpuscular theory failed to adequately explain the diffraction, interference and polarization of light it was abandoned in favour of Huygens' wave theory. To some extent, Newton's corpuscular (particle) theory of light re-emerged in the 20th century, as light phenomenon is currently explained as particle and wave.
Newton's corpuscular theory was an elaboration of his view of reality as interactions of material points through forces. Note Albert Einstein's description of Newton's conception of physical reality:
[Newton's] physical reality is characterised by concepts of space, time, the material point and force (interaction between material points). Physical events are to be thought of as movements according to law of material points in space. The material point is the only representative of reality in so far as it is subject to change. The concept of the material point is obviously due to observable bodies; one conceived of the material point on the analogy of movable bodies by omitting characteristics of extension, form, spatial locality, and all their 'inner' qualities, retaining only inertia, translation, and the additional concept of force.
- Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
- These corpuscles are perfectly elastic, rigid, and weightless.
The fact that light could be polarized was for the first time qualitatively explained by Newton using the particle theory. Étienne-Louis Malus in 1810 created a mathematical particle theory of polarization. Jean-Baptiste Biot in 1812 showed that this theory explained all known phenomena of light polarization. At that time the polarization was considered as the proof of the particle theory.
- Speed of gravity
- Philosophy of physics
- Opticks by Isaac Newton
- The Skeptical Chemist by Robert Boyle
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