Modern Physics

# Newton’s Corpuscular Theory of Light

The Corpuscular Theory of Light Newton (1704)proposes that light material is made up of particles that Isaac Newton called corpuscles. These particles are launched in a straight line and at high speed by different light sources (the Sun, a candle, etc.).

In physics, light is defined as a part of the radiation field called the electromagnetic spectrum. Instead, the term visible light is reserved to designate the part of the electromagnetic spectrum that can be perceived by the human eye. The study of light is responsible for optics, one of the oldest branches of physics.

Light has awakened human interest since time immemorial. Throughout the history of science, there have been many theories about the nature of light. However, it was in the late 17th and early 18th centuries, with Isaac Newton and Christiaan Huygens, when he began to understand his true nature.

In this way they began to lay the foundations for current theories of light. The English scientist Isaac Newton was interested throughout his studies in understanding and explaining the phenomena associated with light and color; The result of his studies he formulated the corpuscular theory of light.

## Newton’s corpuscular theory of light

This theory was published in Newton’s work called Opticks: or, a treatise on reflections, refractions, inflections and colors of light (in Spanish, Optics or treatise on reflections, refractions, inflections and colors of light ).

This theory managed to explain both the straight propagation of light and the reflection of light, although it did not satisfactorily explain refraction.

In 1666, before declaring his theory, Newton performed his famous experiment of decomposing light into color, which was achieved by passing a beam of light through a prism.

The conclusion he reached was that white light is composed of the set of colors in the rainbow, which in his model he explained by saying that the light corpuscles were different depending on the color.

### Reflection

Reflection is the optical phenomenon whereby, when a wave (for example, light) obliquely hits the separation surface between two media, it undergoes a change of direction and is returned to the former along with a part of the energy of the movement.

The laws of reflection are as follows:

#### first law

The reflected ray, the incident, and the normal (or perpendicular) are in the same plane.

#### second law

The angle of incidence value is the same as the angle of reflection. In order for his theory to fulfill the laws of reflection, Newton assumed not only that the corpuscles were very small compared to ordinary matter, but also that they propagated through the medium without suffering friction.

In this way, the corpuscles elastically collided with the
separating surface of the two media and, as the difference in mass was so great, the
corpuscles jumped.

Thus, the horizontal component of momentum px would remain constant, while the normal component p would reverse its meaning.

The laws of reflection were fulfilled, the angle of incidence and the angle of reflection being equal.

### Refraction

On the contrary, refraction is the phenomenon that occurs when a wave (for example, light) obliquely affects the separation space between two media, with different refractive indices.

When this occurs, the wave penetrates and is transmitted through the second medium, along with some of the energy of the movement. Refraction occurs due to the different speed at which the wave propagates in the two media.

An example of the phenomenon of refraction can be observed when an object (eg a pencil or pen) is partially inserted into a glass of water.

To explain refraction, Isaac Newton proposed that luminous particles increase their speed by moving from a less dense medium (such as air) to a denser medium (such as glass or water, for example).

Thus, within the framework of his corpuscular theory, he justified refraction by assuming a more intense attraction of the luminous particles by the medium with greater density.

However, it should be considered that, according to his theory, when a particle of light from air hits water or a glass, it must suffer a force opposite to the component of its velocity perpendicular to the surface, which it would lead to a deviation of the light contrary to what is actually observed.

## Flaws in the corpuscular theory of light

– Newton thought that light travels faster on denser media than on less dense media, which proved not to be the case.

– The idea that the different colors of light are related to the size of the corpuscles is not justified.

– Newton thought that the reflection of light was due to the repulsion between the corpuscles and the surface on which it is reflected; while refraction is caused by the attraction between the corpuscles and the surface that refracts them. However, this statement has been proven incorrect.

It is known that, for example, crystals reflect and refract light at the same time, which, according to Newton’s theory, would imply that they attract and repel light at the same time.

– The corpuscular theory cannot explain the phenomena of diffraction, interference and polarization of light.

## incomplete theory

Although Newton’s theory was an important step in understanding the true nature of light, the truth is that, over time, it turned out to be quite incomplete.

In any case, the latter does not prejudice it as one of the fundamental pillars on which future knowledge about light was built.