Pulse (physics)

From Wikipedia, the free encyclopedia

In physics, a pulse is an individual disturbance that propagates through a transmission medium. This medium may be vacuum (in the case of electromagnetic radiation) or matter, and may be indefinitely large or finite.

Pulse movement and changes can often be described by a partial differential equation (PDE), such as a hyperbolic PDE or a parabolic PDE, which corresponds to the specific type of disturbance.

Free end

Consider a deformation pulse moving through an elastic medium - perhaps through a rope or a slinky. When the pulse reaches the end of that medium, what happens to it depends on whether the medium is fixed in space or free to move at its end. For example, if the pulse is moving through a rope and the end of the rope is held firmly by a person, then it is said that the pulse is approaching a fixed end. On the other hand, if the end of the rope is fixed to a stick such that it is free to move up or down along the stick when the pulse reaches its end, then it is said that the pulse is approaching a free end.

Figure 1: A pulse reaching the end of the medium, the end point is free. The successive positions of the pulse is drawn black, red, green, blue, black, red, green. The final green curve is the initial curve of figure 2.
Figure 2: The reflection of the pulse. The successive positions of the pulse is drawn green, blue, black, red, green, blue. The initial green curve is the final curve of figure 1

A pulse will reflect off a free end and return with the same direction of displacement that it had before reflection. That is, a pulse with an upward displacement will reflect off the end and return with an upward displacement.

This is illustrated by figures 1 and 2 that were obtained by the numerical integration of the wave equation.

Fixed end

Figure 3: A pulse reaching the end of the medium, the end point is fixed. The successive positions of the pulse is drawn black, red, green, blue, black, red, green. The final green curve is the initial curve of figure 4.
Figure 4: The reflection of the pulse. The successive positions of the pulse is drawn green, blue, black, red, green, blue. The initial green curve is the final curve of figure 3
Figure 5: Animation corresponding to figures 3 and 4.

A pulse will reflect off a fixed end and return with the opposite direction of displacement. In this case, the pulse is said to have inverted. That is, a pulse with an upward displacement will reflect off the end and return with a downward displacement.

This is illustrated by figures 3 and 4 that were obtained by the numerical integration of the wave equation. In addition it is illustrated in the animation of figure 5.

Crossing media

When there exists a pulse in a medium that is connected to another less heavy or less dense medium, the pulse will reflect as if it were approaching a free end (no inversion). Contrarily, when a pulse is traveling through a medium connected to a heavier or denser medium, the pulse will reflect as if it were approaching a fixed end (inversion).

Optical pulse

See also

References

Related Articles

Wikiwand AI