What is light? This question has interested mankind in all ages, but only in the 20th century AD did we manage to clarify much about the nature of this phenomenon. This article will discuss the corpuscular theory of light, its advantages and disadvantages.
From philosophers of the ancient world to Christian Huygens and Isaac Newton
Some surviving evidence says that the nature of light began to be interested in ancient Egypt and ancient Greece. At first it was believed that objects emit images of themselves. The latter, falling into the human eye, give the impression of the visibility of objects.
Then, during the formation of philosophical thought in Greece, a new theory of Aristotle appeared, which believed that each person emits some rays from his eyes, thanks to which he can "feel" objects.
The Middle Ages did not bring any clarity to the issue under consideration, new achievements came only with the Renaissance and the revolution in science. In particular, in the second half of the 17th century two completely opposite theories appeared, which sought to explain the phenomena associated with light. We are talking about the wave theory of Christian Huygens and the corpuscular theory of Isaac Newton.
Despite some successes in wave theory, it still had a number of important drawbacks:
- believed that light spreads on the air, which was never discovered by anyone;
- the transverse nature of the waves indicated that the ether was supposed to be a solid medium.
Taking into account these shortcomings, as well as taking into account Newton's enormous authority at that time, the theory of particle-corpuscles was unanimously accepted among scientists.
The essence of the corpuscular theory of light
Newton’s idea is as simple as possible: if all the bodies and processes surrounding us are described by the laws of classical mechanics, in which bodies of finite mass participate, then light also represents small particles or corpuscles. They move in space with a certain speed, if they meet an obstacle, then they are reflected from it. The latter, for example, explains the existence of a shadow in an object. These ideas about the world existed until the beginning of the XIXth, that is, about 150 years.
It is curious to note that Lomonosov used the Newtonian corpuscular theory in the middle of the 18th century to explain the behavior of gases, which is described in his work Elements of Mathematical Chemistry. Lomonosov considered the gas to be composed of particle particles.
What did Newton's theory explain?
The presented ideas about the light took a huge step in understanding its nature. The theory of Newtonian corpuscles was able to explain the following phenomena:
- The rectilinear propagation of light in a homogeneous medium. Indeed, if no external forces act on the moving corpuscle of light, then its state is successfully described by the first Newtonian law of classical mechanics.
- The phenomenon of reflection. Hitting the interface between two media, the corpuscle experiences an absolutely elastic collision, as a result of which its momentum modulus is preserved, and it itself is reflected at an angle equal to the angle of incidence.
- The phenomenon of refraction. Newton believed that penetrating a denser medium from a less dense one (for example, from air to water), the corpuscle accelerates due to the attraction of the molecules of the dense medium. This acceleration leads to a change in its trajectory closer to the normal, i.e., a refraction effect is observed.
- The existence of flowers. The creator of the theory believed that each color observed corresponds to its own "color" corpuscle.
The problems of the theory presented and a return to Huygens's idea
They began to appear when the discoveries of new effects related to light appeared. The main ones are diffraction (deviation from the rectilinear propagation of light when the beam passes through the gap) and interference (the phenomenon of Newton's rings). With the discovery of these properties of light, 19th-century physicists began to recall the work of Huygens.
In the same XIX century, Faraday and Lenz studied the properties of alternating electric (magnetic) fields, and Maxwell carried out the corresponding calculations. As a result, it was proved that light is an electromagnetic transverse wave, which does not require ether for its existence, since the fields that form it generate each other in the process of propagation.
New discoveries related to light and the idea of Max Planck
It would seem that Newton’s corpuscular theory is already completely buried, but at the beginning of the 20th century new results appear: it turns out that light can “tear” electrons out of matter and put pressure on bodies when they fall on them. These phenomena, to which an incomprehensible spectrum of an absolutely black body was added, the wave theory turned out to be powerless to explain.
The solution was found by Max Planck. He suggested that light interacts with atoms of matter in the form of small portions, which he called photons. The photon energy can be determined by the formula:
E = h * v.
Where v is the photon frequency, h is the Planck constant. Max Planck, thanks to this idea of light, laid the foundation for the development of quantum mechanics.
Using the idea of Planck, Albert Einstein explains the phenomenon of the photoelectric effect in 1905, Niels Bohr in 1912 substantiates the atomic emission and absorption spectra, and Compton in 1922 opens the effect that now bears his last name. In addition, the theory of relativity developed by Einstein explained the role of gravity in the deviation from the linear propagation of a light beam.
Thus, the work of these scientists of the early XX century revived Newton's ideas about light in the XVII century.
Particle Wave Theory of Light
What is light? Is it a particle or a wave? During its propagation, whether in the environment or in airless space, light exhibits the properties of a wave. When his interactions with matter are considered, then he behaves like a material particle. Therefore, at present it is customary to talk about light about the dualism of its properties, which are described in the framework of the particle-wave theory.
Particle of light - a photon has neither charge nor mass at rest. Its main characteristic is energy (or frequency, which is one and the same, if you pay attention to the expression above). A photon is a quantum-mechanical object, like any elementary particle (electron, proton, neutron), therefore it has a momentum as if it were a particle, but it could not be localized (to determine the exact coordinates) as if it were a wave.