Which of the following is a direct use of Huygen's Principle? Determining new position of wavefront at a time. Determining the phase of the wavefront after refraction. Determining the phase of wavefront after reflection.

only 1 1 and 2 1 and 3 1, 2 and 3

only 1

On reflection from a denser medium, the phase changes by π. On refraction, the phase remains constant.