Waves are often classified by the way the medium moves in respect to the direction the wave travels. In our example of masses connected by springs, the medium (the masses) moved up and down as the pulse moved from left to right across the screen. This is known as a transverse wave. In a transverse wave, the medium moves perpendicular to the direction of wave propagation. How many possible dimensions could the medium of a transverse wave move? ... There are two transverse directions. We have shown the up and down transverse motion; in and out of the screen is the second direction of motion in transverse waves. Now suppose that instead of moving our spring-connected masses up and down or side to side, we pushed them in and pulled them out. Each mass would move slightly forward and then back. In this case, the masses move in the same dimension as the pulse propagates. This is called a longitudinal wave. In a given wave there is only one dimension of medium motion. When a wave pulse reaches the end of the medium through which it is propagating, it is reflected back. If the end of a rope is fixed so that it cannot move, the pulse flips over and comes back upside down, or inverted. Inverted means turned upside down. If instead of being fixed, the end is free to move up and down, the wave reflects without inverting, propagating back through the medium right side up. If the medium does not end, but simply changes, part of the wave reflects while part continues propagating through, or is transmitted. The reflected wave is inverted when the new medium is harder to move than the original medium. The reflected wave is upright when the new medium is easier to move than the old. The transmitted wave is always upright, regardless of the new medium.

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