**What is wavefront?**

A surface on which all the points have the same phase of vibration is called as wavefront. And wavefront is spherical in shape.

A line normal to the wavefront, showing the direction of propagation of light is called a ray of light.

**History**

The Dutch scientist, Christiaan Huygens (1629-1695) was the person who created a helpful strategy for figuring out exactly how and also where waves propagate. In 1678, he suggested that every point that a disturbance touch, becomes itself a source of a spherical wave.

The sum of the secondary waves (waves that are a result of the disturbance) identifies the kind of the new wave.

Every point on a wave-front may be thought-about a source of additional spherical wavelets which expand in the forward direction at the speed of light. The new wave-front is the digressive surface to all of these second wavelets.

**Huygens’s Principle**

Huygens principle consists of two parts. Starting from a known point, it states that:

- Every point on a wave-front may be considered a source of secondary spherical wavelets which spread out in the forward direction at the speed of light.
- The new wave-front is the tangential surface i.e., the surface that touches all of these secondary wavelets.

**Explanation**

Huygens’ principle provides a practical method to envision refraction. The principle can be described with the formula s=vt, where s is the distance, v is the propagation speed, and t is time.

To determine the wavefront at time t + ∆t, draw secondary wavelets with center at various points on the wavefront. The radius is c ∆t where c is the speed of propagation of the wave. The new wavefront at time t + ∆t is A’B’ which is a tangent envelope to all the secondary wavelets.

**Consider in an easy way!!!**

What happens when you throw a stone in a lake or some small pond? What do you observe? The moving (or better say it oscillating) water… But why it oscillates?

The answer to all of these questions is simple and interesting. When you throw the stone, waves produce and spread in all directions. There are surges formed in the water. The ripples develop the concentric circle around the disturbance and expanded.

These ripples are just the wavefront. The wavefronts gradually spread out in all directions. So, at every factor, we have a wave appearing.

The primary wavefront is created and once again from the main wavefront, a secondary waveform is created, and so forth. The disturbance does not last for a very long time. It fades slowly due to the fact that increasingly more waveforms are developed.

### MCQs about Huygens’s Principle

- What is the definition of wavefront?
- A) A surface with points vibrating in the same phase
- B) A line indicating the direction of light propagation
- C) A point where waves intersect
- D) A surface perpendicular to the direction of wave propagation
**Answer: A) A surface with points vibrating in the same phase**

- Who proposed the concept of Huygens’s Principle?
- A) Isaac Newton
- B) Christiaan Huygens
- C) Galileo Galilei
- D) Albert Einstein
**Answer: B) Christiaan Huygens**

- According to Huygens’s Principle, what happens at every point on a wavefront?
- A) It emits a secondary spherical wavelet
- B) It absorbs incoming waves
- C) It reflects light rays
- D) It remains stationary
**Answer: A) It emits a secondary spherical wavelet**

- What determines the type of new wave according to Huygens’s Principle?
- A) The amplitude of the secondary wavelets
- B) The sum of secondary wavelets
- C) The frequency of the initial wave
- D) The wavelength of the secondary wavelets
**Answer: B) The sum of secondary wavelets**

- How is the new wavefront determined according to Huygens’s Principle?
- A) By averaging the secondary wavelets
- B) By connecting the furthest points of the secondary wavelets
- C) By drawing a tangent surface to the secondary wavelets
- D) By measuring the distance between secondary wavelets
**Answer: C) By drawing a tangent surface to the secondary wavelets**

- What practical application does Huygens’s Principle provide insight into?
- A) Reflection of waves
- B) Refraction of waves
- C) Absorption of waves
- D) Diffraction of waves
**Answer: B) Refraction of waves**

- What formula describes Huygens’s Principle?
- A) E=mc²
- B) F=ma
- C) s=vt
- D) PV=nRT
**Answer: C) s=vt**

- What does ‘s’ represent in the formula s=vt of Huygens’s Principle?
- A) Distance
- B) Time
- C) Speed
- D) Acceleration
**Answer: A) Distance**

- In Huygens’s Principle, what does ‘v’ represent in the formula s=vt?
- A) Volume
- B) Velocity
- C) Vibration
- D) Voltage
**Answer: B) Velocity**

- According to Huygens’s Principle, what determines the radius of secondary wavelets?
- A) The amplitude of the initial wave
- B) The frequency of the initial wave
- C) The speed of propagation of the wave
- D) The wavelength of the initial wave
**Answer: C) The speed of propagation of the wave**

- How are secondary wavelets positioned according to Huygens’s Principle?
- A) In a random pattern
- B) Perpendicular to the primary wavefront
- C) Tangential to the primary wavefront
- D) Opposite to the direction of propagation
**Answer: C) Tangential to the primary wavefront**

- What analogy is used to explain Huygens’s Principle in an easy way?
- A) Throwing a ball
- B) Stirring a cup of coffee
- C) Throwing a stone in water
- D) Blowing air into a balloon
**Answer: C) Throwing a stone in water**

- What gradually happens to the disturbance according to the analogy used for Huygens’s Principle?
- A) It increases in intensity
- B) It forms stationary patterns
- C) It fades away as more waveforms are created
- D) It transforms into a different type of disturbance
**Answer: C) It fades away as more waveforms are created**

- What geometric shape do the wavefronts resemble?
- A) Square
- B) Triangle
- C) Circle
- D) Ellipse
**Answer: C) Circle**

- What term is used to describe the surface touching all secondary wavelets according to Huygens’s Principle?
- A) Tangential surface
- B) Secondary surface
- C) Reflective surface
- D) Absorptive surface
**Answer: A) Tangential surface**

### Wrap up

In conclusion, Huygens’s Principle offers valuable insights into the behavior of waves, particularly regarding the propagation of light.

By defining the concept of wavefronts and illustrating how every point on a wavefront can be seen as a source of secondary spherical wavelets, this principle sheds light on phenomena like refraction. Christiaan Huygens, the Dutch scientist, introduced this principle in 1678, revolutionizing our understanding of wave propagation.

By envisioning waves as emanating from countless secondary sources, we can better grasp their behavior over time and space.

Through practical applications and analogies, such as throwing a stone into water, the principle becomes more accessible, allowing learners to visualize complex wave phenomena more easily.