Introduction to Wave Optics

Newton’s Corpuscular theory

This theory was proposed by Sir Isacc Newton in 1678. According to this theory, known as corpuscular theory, light consists of stream of extremely light and tiny material particles known as corpuscles. These particles are shot out by every point of a source of light with a very high speed.

When the corpuscles, which travel in straight lines, fall on the retina of the eye, they produce the sensation of vision. According to Newton, different colours of light were due to different sizes of corpuscles.

The theory could explain the propagation of light through vacuum and phenomenon of reflection and refraction. But the theory could not explain the phenomena of interference, diffraction and polarisation. It could not explain why velocity of light is lesser in a denser medium compared to vacuum.

Origin of the wave theory

As only few physical phenomenon could be explained by corpuscular theory, Christian Huygens proposed the wave aspect of light. According to which, a luminous body is a source of disturbance in a hypothetical medium called ether. The medium assumed to be spread in the entire space. The disturbance from the source is propagated in the form of waves through ether and the energy is distributed equally in all directions. Huygens assumed these waves to be longitudinal in which the vibration of particle of the medium is parallel to the direction of propagation of the wave.

Huygens successfully explained the linear propagation of light, phenomenon of reflection, refraction and double refraction. However, the phenomenon of polarisation discovered by him could not be explained.

Later, Fresnel and Young suggested that light waves are transverse.

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