Huygens Principle of Secondary Wavelets

Huygen’s theory talks about the wave nature of light. Huygen found an error with Newton’s statement. On observing the spreading of light through a dark room when passing through an obstacle, the intensity of light decreases with the distance of the light path. Thus, Huygen proposed that light has a wave nature, which finally concluded that light possesses both wave and particle nature. On advancing the proposals further, Huygen’s principle of secondary wavelets talks about the secondary waves originating and moving from the primary ones. The examples are mentioned in Huygen’s principle of secondary wavelets.

Wavefront

• Consider a vibration at a constant frequency

• The waves of the vibration move in the direction of propagation

• There is a set of points or locus that are at the same phase or in unison with each other

• These points are the wavefronts of the waves

Wavefront types

There are three types of wavefronts. They are as follows.

1. Spherical wavefront

• The wavefronts are spherical like exact circles

• They form wavelets being visible as concentric circles of waves

• For example, ripples in the water due to the disturbance of a pebble

2. Planar wavefront

• The planar wavefronts are like a minute portion of the spherical wavefronts

• It passes as linear waves

• For example, the sunlight wavefronts

3. Cylindrical wavefronts

• Cylindrical wavefronts are cylindrical

• They have equally distanced horizons at both the sides of the secondary wavelets from the centre of the waves

• For example, cylindrical wavefronts form when a wave is passed through a narrow slit

Huygen’s Principle 

• Huygen’s principle was put forward by Christiaan Huygens and Augustin-Jean Fresnel

• Thus, Huygen’s principle of secondary wavelets is also referred to as the Huygens-Fresnel principle

• It deals with the motion of the new waves that are formed as secondary from the previously originated ones

Huygen’s principle of secondary wavelets: statement

• Huygen’s principle of secondary wavelets portrays that the secondary wavelets or the secondary disturbances originate from any given possible wave point

• These small secondary ripples spread and progress forward with the speed of the parent one

• At any given wavefront point, a novel wavelet appears and spreads ahead in the forward direction of the perpendicular drawn to the previous wavefront

• By employing Huygen’s principle of secondary wavelets, we can predict the position of the newly formed wavelet

• The time taken for the new wavelets to form from the primary wavefronts is constant for every set of wavelets

Huygens principle of secondary wavelets: examples

Huygens-Fresnel principle not only tells that a new wavefront forms from the previously existing one but also, paves a way to predict the position of the new wavelet. For easy understanding in this Huygens principle of secondary wavelets UPSC notes, the following example would help.

In water

• Consider dropping a pebble or splashing the water once at a single point with your palm

• Have you ever noticed the consistent circles that form in the water after such disturbance?

• That depicts the movement of waves and the formation of wavelets

• As soon as the pebble drops into the water, the first circle of the wave is formed moving outwards from the point of the pebble contact. It is considered as the forward direction

• This originated wave disturbs the nearby water surface and forms a new set of wavelets that are tangential to the primary wavefront

• These newly originated waves are the secondary waves or secondary disturbances

• The secondary wavelets move in the forward direction, too, the same as the primary wave

• The secondary wavelet originates in the forward direction, along the perpendicular line drawn at any given wavefront point

• At each wavefront, a semi-circular wave originates

• Thus, a set of concentric circles of wave disturbances rises with the speed of the primary wavefronts

• All these wavelets slowly settle after some time.

For sound

• Imagine a song playing inside a room

• Applying Huygens principle of secondary wavelets to sound waves, you will not hear the song clearly until you stand in front of the doorway

• It is because the newly formed wavelets have no way to purse across the wall

• Thus, the wavelets escape only through the doorway and the song is heard only when standing in front of the doorway

For light

• In the case of light waves, the light intensity reduces when it travels across an obstacle like a small opening

• Thus, the secondary wavelets are prevented from moving by the obstacle that is placed there

• The wavelets move ahead only through the open slit

Conclusion

Huygen’s principle of secondary wavelets shows that the secondary wavelets originate from the previously formed wavefront. The new wavelets move in the forward direction along the tangential line drawn at any point in the wavefront.

Huygen’s principle of secondary wavelets details the examples and significance of the principle. Huygen’s principle of secondary wavelets examples includes ripples in the water due to pebble disturbance, propagation of light through a slit into a dark room, decreased intensity of sound waves, travelling through a narrow opening, etc.