CHAPTER 17 : NATURE OF LIGHT
Light as a form of energy is a widely known fact. While discussing its nature, an important question arises.
Is it made up of particles or is it a wave propagation form of energy ? The modern view point is that
light is a wave particle duality. However, for the present analysis only the theories prevalent upto the end of
the 19th century will be considered.
17.1 Newton's Corpuscular theory
Newton considered light as made up of particles called Corpuscules. The corpuscles of light were
assumed to be almost point like particles without any mass.
These assumptions were made to account for the rectilinear
propagation of light. To explain reflection and refraction of light
Newton claimed that the boundaries exert repulsive and attractive
forces respectively on the light Corpuscles. Thus laws of motion
were made applicable to light Corpuscles.
However this theory is unacceptable because :
(1) Simultaneous reflection and refraction at a semi-transparent boundary will require both
repulsive and attractive forces from the boundary to the light corpuscles, which is clearly an absurdity.
(2) The speed of light should be greater in a denser medium than in a rarer medium; this prediction
is contrary to experimental results.
(3) The phenomena of interference, diffraction and polarization cannot be explained at all.
Newton was aware of the dispersion of light and had also produced an interference pattern known
as Newton's rings. However his chief objection to the wave theory was that the rectilinear propagation
of light would not be possible if light was a wave propagation since diffraction of light would occur in
this case. If only he had known that wave length of light is of the order of
10-5 cm, his objections would have melted away easily.