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| Formation of Waves |
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| One of the familiar types of waves is the wave on the surface of water. Let a pebble (P) be dropped gently on the calm surface of water in a pond. Before the pebble is dropped the surface of water is flat. When the pebble falls, it pushes the water under it, downwards. Water, being practically incompressible, gets displaced and rises up all-around in a ring. The pebble passes through water and reaches the bottom of the pond. |
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| The water at P1 and P2 in trying to go back to its equilibrium position, overshoots this position and goes below the normal level of water. This makes the neighbouring water at P3, P and P4 to move up. When the water at P3, P and P4 tries to go back to the equilibrium position, it again overshoots this position. This makes the water level to rise up at P5 and P6. Thus, the ripples spread outwards in circles of increasing radii, with P as centre. |
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| Let one end of a long rope be attached to a rigid support and the other end held in the hand. If the rope is jerked suddenly, by moving the hand in a single up and down motion as in figure(a) below, an impulse is passed along the rope from particle to particle. A hump formed near the hand, called the wave pulse, travels along the rope. The chief characteristic of a wave pulse is that it has a beginning and an end. It is a disturbance of limited extent. At any instant, a limited region of space is disturbed. The wave pulse passes by any point in a limited time. We have assumed an “ideal” rope in which there are no friction-like forces to cause the wave to die down as it travels along. Further, we have assumed that the rope is so long that the reflection or the echo of the wave from the far end need not be considered. If the hand is moved up and down in continuous simple harmonic motion as in the figure (b) an extended travelling sinusoidal wave (shape resembling a sine curve) is generated. |
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| Let a long spring (also called a slinky) be placed horizontally on a smooth floor with one end of it resting against a wall. On giving a sudden flip to the free end of the slinky so as to compress it, a compressional wave is found to pass down its length. The few rings near the free end which get compressed due to the flip, bounce back. But the energy is already passed on to the other rings. By frequently flipping the free end, compressional pulses can be formed all along the length of the slinky. |
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| From the above illustrations, it is clear that the particles of the medium move over short paths about their initial positions. As a result, a wave moves through the medium. In the above cases, the motion of the wave through the medium is a result of the action of the successive parts of the medium on one another. Hence, such a wave can travel only in an elastic medium. |
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