Huygen's principle is applied to

Explanation

Huygen's principle is a fundamental concept in wave optics that helps determine the position and shape of wave fronts as they propagate through space. According to this principle, every point on a wave front acts as a source of secondary spherical wavelets, and the new wave front is the tangent to these wavelets. This principle is primarily applied to locate the wave front, which is why option A is correct.Options B and C refer to phenomena explained by wave theory, but they are not the direct applications of Huygen's principle. Option D is incorrect because Huygen's principle does not address the speed of light but rather the behavior of wave fronts.