State Huygens principle of secondary wavelets.
Kindly refer the following link:
https://www.meritnation.com/cbse/class12-science/studymaterial/physics/physics-part-ii/wave-optics/216_254_8669#slide1_huygens -principle
https://www.meritnation.com/cbse/class12-science/studymaterial/physics/physics-part-ii/wave-optics/216_254_8669#slide1_huygens -principle