Huygen's principle describes how each point on a wavefront acts as a secondary source of waves, and that the new wavefront is the envelope of these secondary waves. It provides a method for determining the propagation and behavior of light waves. The principle is based on assuming each point on the initial wavefront emits spherical wavelets, and the new wavefront is the envelope of these secondary waves. It can be used to understand phenomena like interference and diffraction of light.

1. Huygen’s Principle

2. WAVEFRONT
The locus of all the particles of the medium, which at
any instant are vibrating in the same phase, is called
the wavefront.
Depending upon the shape of the source of light,
wavefront can be of the following types:
Spherical wavefront.
Cylindrical wavefront.
Plane wavefront.

3. Ray of light:
An arrow drawn normal to the wavefront and
pointing in the direction of propagation of
disturbance represents a ray of light.

4. HUYGENS’ PRINCIPLE
Huygens’ principle is a geometrical construction, which is
used to determine the new position of a wavefront at a later
time from its given position at any instant. In other words,
the principle gives a method to know as to how light spreads
out in the medium.
Huygens’ principle is based on the following assumptions:
Each point on the given or primary wavefront acts as a source
of secondary wavelets, sending out disturbance in all directions
in a similar manner as the original source of light does.
The new position of the wavefront at any instant (called
secondary wavefront) is the envelope of the secondary wavelets
at that instant.
The above two assumptions are known as Huygens’
principle or Huygens’ construction.

5. INTERFERENCE OF LIGHT
The phenomenon of non-uniform distribution of energy
in the medium due to superposition of two light waves is
called interference of light.
COHERENT SOURCES
Two independent sources of light cannot emit waves
continuously.
The waves emitted by two independent sources of light
do not have same phase or a constant phase difference.

6. PRINCIPLE OF SUPERPOSITION OF LIGHT WAVES
YOUNG’S DOUBLE SLIT EXPERIMENT
PRINCIPLE OF SUPERPOSITION OF LIGHT WAVES

12. TRANSVERSE NATURE OF LIGHT
Longitudinal waves: The waves, in which the particles oscillate
along the direction of propagation of the waves, are called
longitudinal waves.
Transverse waves: The waves, in which the direction of oscillation
of particles is perpendicular to the direction of propagation of the
waves, are called transverse waves.

14. LAW OF MALUS
It states that when a completely plane polarized light
beam is incident on an analyser, the intensity of the
emergent light varies as the square of the cosine of the
angle between the planes of transmission of the analyser
and the polarizer.

15. TRANSVERSE NATURE OF LIGHT
Longitudinal waves: The waves, in which the particles
oscillate along the direction of propagation of the waves,
are called longitudinal waves.
Transverse waves: The waves, in which the direction of
oscillation of particles is perpendicular to the direction of
propagation of the waves, are called transverse waves.