Corpuscular Theory - Class 11 Pysics PDF Download

Corpuscular Theory Definition

The corpuscular theory states that light is made up of tiny particles called ‘corpuscles’ (little particles) that always travel in a straight line.

What is Corpuscular Theory?

Light is a form of energy that travels from one place to another place at high velocity. Various scientists have attempted to explain the nature of light. However, the first scientific attempt to explain the nature of light was made by Sir Isaac Newton.

In the 17^th century, two different theories about the nature of light were proposed; these theories were the ‘wave theory’ and ‘corpuscular theory’.

The ‘Corpuscular theory of light was proposed by Newton in 1704. In this theory, he successfully explained the nature of light.

The corpuscular theory is the simplest theory of light in which light is assumed as the tiny particles called ‘corpuscles’. The corpuscular theory is often referred to as particle theory or Newton’s theory of light.  

Newton’s Corpuscular Theory Statement

According to the Newton’s corpuscular theory:

• Light is made up of tiny particles called ‘corpuscles’ having negligible mass.
• These particles (corpuscles) are perfectly elastic.
• The corpuscles are emitted from the luminous sources such as Sun, candle, electric lamp etc.
• The tiny particles (corpuscles) always travel in a straight line in all directions.
• Each particle (corpuscle) carries kinetic energy with it while moving.
• The corpuscles travel at high velocity.
• The corpuscles (light) would travel faster in the denser medium than in rarer medium. But later this is proved wrong. We know that light travels faster in the rarer medium than in denser medium.
• When the particles (corpuscles) fall on the retina of the eye, they produce an image of the object or sensation of vision.
• The corpuscles can be of different sizes. The different colors of light are due to the different sizes of the corpuscles.
  
  
  Corpuscular theory example

The corpuscular theory could be easily understood with a simple example.

• Take a rubber ball.
• Stand before a wall.
• Throw the ball in such a way that it hits the wall normally.

• You will observe that the ball bounces and returns along the same path.
• Now, throw the ball in such a way that it makes a certain angle with the surface of the wall.

• You will observe that the ball bounces and goes away from the surface of the wall making an equal angle with respect to the normal drawn at the point of incidence.

These observations show that the rubber ball obeys the laws of reflection. Just like the rubber ball, when corpuscles hit the surface they are reflected back in the same path or reflect at an equal angle with respect to the normal drawn at the point of incidence.

The corpuscles of light behave like the rubber balls.

Corpuscular theory about reflection of light

The corpuscular theory explains the reflection of light in exactly the same way as the reflection of a perfectly elastic ball from a rigid plane.

When the corpuscles (particles) hit the reflecting surface, they are reflected from it in such a way that the angle of incidence is equal to the angle of reflection. This is due to the repulsion between the corpuscles and the reflecting surface.

Corpuscular theory about refraction of light

According to Newton, when corpuscles (light particles) approach the refracting surface, they are attracted near the surface. When they enter the denser medium from a rarer medium, their speed increases and hence change their direction.

Failures of Newton’s corpuscular theory

• Newton assumed that light travels faster in the denser medium than in the rarer medium which has been proved wrong.
• The assumption that the different colors of light are due to the differences in the sizes of corpuscles has no justification.
• Newton assumed that reflection of light is due to the repulsion between the corpuscles and reflecting surface while refraction of light is due to the attraction between the corpuscles and refracting surface. Thus, a medium could either attract or repel the corpuscles of light. But later this statement was proved wrong. We all know that glass partially reflects and partially refracts light. How could then glass attract and repel the corpuscles of light simultaneously.
• Corpuscular theory cannot explain the phenomena of diffraction, interference, and polarization of light.