B

Step 1: Understand Huygen's principle.
Huygen's principle states that every point on a wavefront can be considered as a source of secondary wavelets, which spread out in all directions at the same speed as the wave.
Step 2: Application of the principle.
By constructing new wavefronts using these secondary wavelets, we can determine the position of the wave front at any future time. This is indeed a geometrical method used to find a future wavefront.
Step 3: Evaluate each option given.
- Option A is incorrect because while Huygen's principle can relate to focal lengths, it does not specifically allow us to find the focal length of a thick lens.
- Option C is incorrect since it does not directly provide a method to determine the velocity of light.
- Option D is incorrect because Huygen's principle does not explain polarization, which involves the orientation of light waves.
Therefore, the correct answer is B.