Huygens’s Principle

Huygens’s Principle

Huygens’s construction:

Huygens’s construction is a geometrical method which is used to determine the position and nature of given wavefront at a later instant. Huygen’s construction is based on the Huygen’s principle


Huygen’s Principle:

Each and every point on a wavefront acts as a secondary source of light. They will emit secondary waves in all possible directions with the same velocity in the same medium. The tangential surface which touches all the secondary wavelets, at later instant is position of the new wavefront at that instant.


Huygen’s Construction of a spherical wavefront and plane Wavefront

Huygen’s construction of a spherical wavefront:

Consider a point source S of light, placed at a point in vacuum or in air medium. The wavefront originating from the point source of light at finite distance is spherical wavefront. Let PQR be the initial position of the spherical wave front. According to Huygen’s principle, each and every point on the wavefront PQR acts as secondary source of light and emits secondary waves in all possible direction with the same velocity in the same medium. Let ‘v’ be the velocity of light in air medium. Hence to determine the position of new wavefront after time‘t’, draw spheres assuming each and every point of wavefront PQR as its centre and vt as its radius. The tangential surface P`Q`R` which touches all spheres in position of the new wavefront after time ‘t’. From figure it appears that there is a back wavefront, but the waves moving in backward direction does not exist.

Huygen’s construction of a plane wavefront:

Consider a point source ‘S’ of light placed at a point in vacuum or in air medium. The wavefront originating from the point source of light at very large distance is plane wavefront.
Let PQR be the initial position of the wavefront. According to huygen’s principle each and every point of the wavefront PQR acts as secondary source of light and emits secondary waves in all possible direction with the same velocity in the same medium. Let ‘v’ be the velocity of light in air medium. To determine the position of new wavefront after time ‘t’ draw spheres assuming each and every point of wavefront PQR as its centre and vt as its radius. The tangential surface P`Q`R` which touches all spheres is the position of new plane wavefront after time t. From figure it appears that there is a back wavefront but the waves moving in backward direction does not exist.