Question
State Huygens' principle. Explain geometrical construction of a plane wavefront.
✓
Answer
According to Huygens principle, each point of the wavefront is the source of a secondary disturbance and the wavelets emanating from these points spread out in all directions with the speed of the wave. These wavelets emanating from the wavefront are usually referred to as secondary wavelets and if we draw a common tangent to all these spheres, we obtain the new position of the wavefront at a later time.
a. A plane wavefront is formed when point of observation is very far away from the primary source.
b. Let PQR represent a plane wavefront at any instant. According to Huygens' principle,all the points on this wavefront will act as secondary sources of light sending out secondary wavelets in the forward direction.
c. Draw hemispheres with $P, Q R$.... as centres and 'ct' as radius. The surface tangential to all such hemispheres is $P_1 Q_1 R_1 \ldots$. at instant ' $t$ '. It is a new wavefront at time ' $t$ '.
d. The plane wavefronts is propagated as plane waves in homogeneous isotropic medium. They are parallel to each other.
e. PP1N1, $Q Q_1 N_2, R R_1 N_3$ are the wave normals at $P, Q R$ respectively. These wave normals show the direction of propagation of plane wavefront.
f. The new wavefront $P_1 Q_1 R_1$ is parallel to primary wavefront PQR.
a. A plane wavefront is formed when point of observation is very far away from the primary source.
b. Let PQR represent a plane wavefront at any instant. According to Huygens' principle,all the points on this wavefront will act as secondary sources of light sending out secondary wavelets in the forward direction.
c. Draw hemispheres with $P, Q R$.... as centres and 'ct' as radius. The surface tangential to all such hemispheres is $P_1 Q_1 R_1 \ldots$. at instant ' $t$ '. It is a new wavefront at time ' $t$ '.
d. The plane wavefronts is propagated as plane waves in homogeneous isotropic medium. They are parallel to each other.
e. PP1N1, $Q Q_1 N_2, R R_1 N_3$ are the wave normals at $P, Q R$ respectively. These wave normals show the direction of propagation of plane wavefront.
f. The new wavefront $P_1 Q_1 R_1$ is parallel to primary wavefront PQR.
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