Huygen’s principle :
According to it :
(i) Every point on the given wave-font (called primary wave front) acts as a fresh source of new disturbance, called secondary wavelets, which travel in all directions with the velocity of light in the medium.
(ii) A surface touching these secondary wavelets, tangentially in the forward direction at any instant gives the new wave-front at that instant. This is called secondary wave-front.
