1. Huygens’s Principle
Dutch scientist Christian Huygens (1629–1695) developed a principle that is useful in explaining wave phenomena.
Huygens’principle describes how a wave front that exists at one instant gives rise to the wave front that exists later on.
This principle states that: Every point on a wave front acts as a source of tiny wavelets that move forward with the same speed as the wave; the wave front at a later instant is the surface that is tangent to the wavelets.

2. Huygens’s Wavelets

3. The Principle of Linear Superposition
When two or more waves are present simultaneously at the same place, the resultant disturbance is the sum of the disturbances from the individual waves.
The superposition principle can be used to explain interference of waves.

4. Constructive Interference of Sound Waves

5. Destructive Interference

6. Inverse Square Law

7. Example on Inverse Square Law

8. Polarization
One of the essential features of electromagnetic waves is that they are transverse waves, and because of this feature they can be polarized.
The figure illustrates the idea of polarization by showing a transverse wave as it travels along a rope toward a slit.
The wave is said to be plane polarized, which means that its vibrations always occur along one plane.
This plane is called the plane of polarization.

9. The Nature of Electromagnetic Waves
The changing magnetic field creates an electric field that fluctuates in time and the changing electric field creates the magnetic field.

10. POLARIZED ELECTROMAGNETIC WAVES

11. Polaroid sunglasses
When Polaroid sunglasses are uncrossed (left photograph), the transmitted light is dimmed due to the extra thickness of tinted plastic.
However, when they are crossed (right photograph), the intensity of the transmitted light is reduced to zero because of the effects of polarization.

12. THE OCCURRENCE OF POLARIZED LIGHT IN NATURE
In the process of being scattered from atmospheric molecules, un-polarized light from the sun becomes polarized.