Huygens Principle (Huygens-Fresnel Principle)

What is light? In short, light is the electromagnetic radiation that can be detected by the human eye. Light has the wave model, ray model, and photon model. Huygens principle in optics is associated with the wave nature of light. This principle is also known as Huygens-Fresnel Principle. This helps us in predicting and understanding the classical wave propagation of light. Huygens Principle can be used in radar waves as well.

In 1678 Christiaan Huygens proposed a principle that helped us to understand the light and light's nature more. This principle states that,

Every point on a wavefront is in itself the source of spherical wavelets which spread out in the forward direction at the speed of light. The sum of these spherical wavelets forms the wavefront.

What is Wavefront?

A wavefront refers to the locus of all the points of the medium which are vibrating in phase and are also displaced at the same time. 

Example of wavefront: If we throw a stone into a lake, then it will create waves around the point the stone has been thrown in the form of ripples that are circular in nature. This called a wavefront. 

According to this principle

If we want to analyze this a bit, then we will find that all points on the primary wavefront are the source of the secondary disturbance. These secondary waves travel through space with the same velocity at the original wave and the envelope of all the secondary wavelets after any given interval of time gives rise to the secondary wavefront. 

Advantages of Huygens Principle

1. The concepts of diffraction of light and interference of light were proved by this principle.
2. It also proved the reflection and the refraction of light.

Limitations of Huygens Principle

1. This principle cannot explain the photoelectric effect.
2. The absorption of light cannot be explained by this.
3. Polarization of light and emission of light was not explained by Huygens Principle.
4. This theory proposes an all-pervading medium required to propagate light called luminiferous ether. But this was proved wrong in the 1900s. 

FAQs:

1. What is light?

Light is the electromagnetic radiation that can be detected by the human eye.

2. Which nature of light cannot be explained by the Huygens Principle?

The photoelectric effect, the absorption of light, the polarization of light, and emission of light cannot be explained by Huygens Principle.

Practice Problems

1. The Huygens principle is also known as the-

a) Huygens-Bose Principle

b) Huygens-Fresnel Principle

c) Huygens-Einstein Principle

d) Huygens-Newton Principle

Correct answer: b

2. Which of the effect cannot be explained by the Huygens Principle?

a) Magnetic Effect

b) Reflection of light

c) Electromagnetic Effect

d) Photoelectric effect

Correct answer: d