A parallel beam of light strikes a piece of transparent glass having cross section as shown in the figure below. Correct shape of the emergent wavefront will be (figures are schematic and not drawn to scale)-

Explain how to get a new wavefront in time $\pi $ using Huygen's principle for plane wavefront.

Write Huygen's principle and principle basically composited ? 

Write the limitation of Huygen's principle. 

Carefully analyses the diagram and choose the correct option :

What is the most important point of Huygen's wave theory ?