17.2 Huygen's Wave Theory of Light
Around the same time, as Newton formulated his corpuscles' theory, his contemporary Huygen
formulated the wave theory of light. The predominant model of wave propagation upto the end of the
19th century was modulations in some medium, therefore a medium called ether was hypothesized for waves
of light to be produced due to vibrations of the particles of ether. The necessity of hypothesizing ether
which is different than any material medium known to us, arises because light travels through empty space
for example from sun and stars as well as dense objects like glass, diamond etc.
The following constructions were proposed by Huygen's to explain the wave propagation of light.
Primary source : The geometrical center or axis of the actual source of light which is either a point or
a line is called the primary source.
Wavelets : All points lying on small curved surfaces, that receive light at the same time from the
same source (primary or secondary) are called wavelets.
Secondary source :Any point on a wavelet, acts as the source of light for further propagation of
light. It is called a secondary source.
Wave front : The envelope of all wavelets in the same phase- having received light from sources in
the same phase at the same time is called a wave front.
Wave normal : The normal at any point drawn outward on
a wave front is called the wave normal. Further propagation of light
occurs along the wave normal. In isotropic media the wave normal
coincides with the 'ray of light'.
Spherical, cylindrical and plane wavefronts
The wave fronts from a point of the primary source are spherical wave fronts. The wavefronts from
the line primary source are cylindrical wave fronts.
At large distances from the primary source the radius of curvature of the wave fronts is negligibly
small. Small portions of such wave points are essentially flat or plane. These are called plane wave fronts.
It should be noted that light propagates by way of expansion of wave fronts.
The above constructions are illustrated in Figure 1.