Study Material

# Newton’s Corpuscular Theory of Light

## What is corpuscular theory?

Newton’s corpuscular theory of light (1704) proposes that light is composed of material particles which Isaac Newton called corpuscles. These particles are thrown 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. Optics, one of the oldest branches of physics, is responsible for the study of light.

Light has aroused 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 seventeenth and early eighteenth centuries, with Isaac Newton and Christiaan Huygens, that its true nature began to be understood. Newton’s Corpuscular Theory of Light

Thus began to lay the foundations for current theories about light. The English scientist Isaac Newton was interested throughout his studies to understand and explain the phenomena associated with light and colors; As a 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 of the Reflexions, refractions, inflexions and colors of light (in Spanish,  optically or treaty reflections, refractions, inflections and colors of light ).

This theory was able to explain both the rectilinear propagation of light and the reflection of light, although it did not satisfactorily explain refraction.

In 1666, prior to enunciating his theory, Newton had carried out his famous experiment of decomposition of light into colors, which was achieved by making a beam of light pass through a prism. Newton’s Corpuscular Theory of Light

The conclusion he reached was that white light is composed of all the colors of the rainbow, which in his model he explained by saying that the corpuscles of light were different depending on their color. Newton’s Corpuscular Theory of Light

### Reflection

Reflection is the optical phenomenon whereby when a wave (for example, light) falls obliquely on the separation surface between two media, it undergoes a change of direction and is returned to the first 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 value of the angle of incidence is the same as that of the angle of reflection. In order for his theory to comply with the laws of reflection, Newton assumed not only that the corpuscles were very small compared to ordinary matter , but that they also propagated through the medium without suffering any kind of friction. Newton’s Corpuscular Theory of Light

In this way, the corpuscles would collide elastically with the
separation surface of the two media, and since the difference in masses was very large, the corpuscles would bounce.

Thus, the horizontal component of the momentum  px would remain constant, while the normal component p would reverse  its direction. Newton’s Corpuscular Theory of Light

Thus 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) falls obliquely on the separation space between two media, with different refractive indexes.

When this happens, the wave penetrates and is transmitted for a half second along with a part of the energy of the movement. Refraction takes place 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 (for example, a pencil or a pen) is partially inserted into a glass of water.

To explain refraction, Isaac Newton proposed that light particles increase their speed as they move from a less dense medium (such as air) to a denser medium (such as glass or water).

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

However, it must be considered that, according to his theory, at the instant in which a luminous particle from air strikes water or glass, it should undergo a force opposite to the component of its velocity perpendicular to the surface, which it would entail a deviation of the light contrary to that actually observed.

## Failures of the corpuscular theory of light

The corpuscular theory of light has several errors:

• Newton believed that light travels faster in denser media than in less dense media, which has been shown not to be the case.
• The idea that the different colors of light are related to the size of the corpuscles has no justification.
• 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 was found to be 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 proved quite incomplete. Newton’s Corpuscular Theory of Light

In any case, the latter does not detract from its value as one of the fundamental pillars on which future knowledge about light was built. Newton’s Corpuscular Theory of Light