Name: 靜止的波浪與動畫課程 (Standing (Stationary) Waves Lesson with Animation)
Uploaded: 2021-01-14T05:56:10.000Z
Duration: 1 min 25 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

We are going to explain the reflection of electromagnetic waves (is equivalent for acoustic waves)

caused by a mismatch on the impedance value of the propagation medium

We suppose an incident wave travelling in the +x axis following a sinusoidal form (blue)

When the incident wave arrives to the end of the medium, we must calculate the reflection coefficient, which depends on the relationship of the two medium impedances.

If this reflection coefficient is equal to -0.35 the reflected wave (red) is going to have an amplitude 0.35 times the amplitude of the incident wave and a phase shift of 180 degrees.

The resulting (total, stationary) wave is the sum of the two waves (green)

Now we animate the two waves and we see what really happens with a set of different reflection coefficients

For a reflection coefficient of -1 all the incident wave is reflected with a phase shift of 180 degrees

This causes that the stationary wave takes value from 0 to 2 times the incident wave peak)

As we can see the positions of the maximums and minimums (nulls) are fixed

For a reflection coefficient of -0.5 half of the incident wave is reflected with a phase shift of 180 degrees

Now the interference of the two waves don't cause nulls and maximums with two times the incident wave peak value

For a reflection coefficient of 0 the mismatch doesn't exist, so there isn't reflected wave and the incident wave is directly the stationary wave (blue and green waves are overlapping)

Of course the maximum value is the same across all the distance

For a reflection coefficient of 0.5 half of the incident wave is reflected with no phase shift between them.

Is equivalent to the -0.5 case. The unique changes are the positions of maximum and minimum values of the stationary wave.

For a reflection coefficient of 1 all the incident wave is reflected with no phase shift between them.

Is equivalent to the -1 case. The unique changes are the positions of maximum and minimum values of the stationary wave.