what is huygens’s priciple?Explain the optical phenomenon of refraction using Huygen’s principle?

In 1678 the great Dutch physicist Christian Huygens (1629-1695) wrote a treatise called Traite de la Lumiere on the wave theory of light, and in this work he stated that the wavefront of a propagating wave of light at any instant conforms to the envelope of spherical wavelets emanating from every point on the wavefront at the prior instant (with the understanding that the wavelets have the same speed as the overall wave). An illustration of this idea, now known as Huygens' Principle, is shown below.



This drawing depicts the propagation of the wave “front”, but Huygens’ Principle is understood to apply equally to any locus of constant phase (not just the leading edge of the disturbance), all propagating at the same characteristic wave speed. This implies that a wave doesn't get "thicker" as it propagates, i.e., there is no diffusion of waves. For example, if we turn on a light bulb for one second, someone viewing the bulb from a mile away will see it "on" for precisely one second, and no longer. Similarly, the fact that we see sharp images of distant stars and galaxies is due to Huygens’ Principle. However, it’s worth noting that this principle is valid only in spaces with an odd number of dimensions. (See below for a detailed explanation of why this is so.) If we drop a pebble in a calm pond, a circular wave on the two-dimensional surface of the pond will emanate outward, and if Huygens' Principle was valid in two dimensions, we would expect the surface of the pond to be perfectly quiet both outside and inside the expanding spherical wave. But in fact the surface of the pond inside the expanding wave (in this two-dimensional space) is not perfectly calm, its state continues to differ slightly from its quiescent state even after the main wave has passed through. This excited state will persist indefinitely, although the magnitude rapidly becomes extremely small. The same occurs in a space with any even number of dimensions. Of course, the leading edge of a wave always propagates at the characteristic speed c, regardless of whether Huygens' Principle is true or not. In a sense, Huygens' Principle is more significant for what it says about what happens behind the leading edge of the disturbance

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