THE PHYSICS OF TSUNAMI AND WAVES

1. Why the circular waves in the pond goes on expanding?
2. When you happened to be in a room, why all the outside noise appears to came from the door step?
3. Why tsunami affects only some parts of the land?
4. When any wave pass through a gap, it spread out in semi-circular fashion, why?
     All these questions are answered by "Huygen's principle".  What is that?

     Say, you drop a stone in a placid sheet of water.  Circular waves are created and starts expanding.  Huygen's principle says, "every point in a circular wave acts as a secondary source of waves.  And new wavelet starts from these points and move forward (as if a stone is dropped at every point of the circle).  The new wave front is the envelop (collection) of these wavelets".   This principle is a good tool (not a law) to predict the path and the progress of waves.  Now we can answer the above questions.:

1. Since every point on a wavefront act as source; the  new wavelets always arises; and new wavefronts always created. The points in the new wavefront again act as sources.  Secondary wavelets are emitted.  Yet again another wavefront is formed.  This goes on repeating.
2. When the sound waves from the street reaches the entrance of a room, new wavelet starts from there and reaches your ear.  These sound waves carry no history of its path.  Hence it appears that the sound is originating from the entrance.
3. The big Tsunami waves silently move on the surface of the ocean.  When it encounters island, or islets, it spread through them in all directions (due to Huygen's principle).  Hence, in some direction, its energy is dissipated and in some other directions, it hit the land with ferocity, when Tsunami hit Tamil-nadu-India, mostly northern state was affected.  But the southern part of the state was protected by the island Srilanka.

4. Imagine a big lake and a small pond separated by a wall.  Say, there is a small gap in the wall.  When a wave from the lake hit the opening in the wall, it pass through it and expand (spread in all directions) in the pond, because of Huygen's principle.

    This principle also explains reflection, refraction, interference and diffraction.  This principle also implies that two waves can travel through each other.
     Today we know, sound and water waves are pressure waves.  The light is electromagnetic wave.  But still, this principle is useful tool to study the progression of waves.
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