Consider the diffraction pattern for a small pinhole. As the size of the hole is increased
For light diverging from a point source,
Is Huygen's principle valid for longitudinal sound waves?
When we are considering a point source of sound wave. The disturbance due to the source propagates in spherical symmetry that is in all directions. The formation of wavefront is in accordance with Huygen's principle.
So, Huygen's principle is valid for longitudinal sound waves also.
Consider a point at the focal point of a convergent lens. Another convergent lens of short focal length is placed on the other side. What is the nature of the wavefronts emerging from the final image?
Consider the ray diagram shown below
The point image $I_1$, due to $L_1$ is at the focal point. Now, due to the converging lense $L_2$, let final image formed is $I$ which is point image, hence the wavefront for this image will be of spherical symmetry.
What is the shape of the wavefront on earth for sunlight?
We know that the sun is at very large distance from the earth. Assuming sun as spherical, it can be considered as point source situated at infinity.
Due to the large distance the radius of wavefront can be considered as large (infinity) and hence, wavefront is almost plane.
