Standing Waves: Incident and Reflected Waves' Equal Frequency

[jeehoo]

Homework Statement

How does a standing wave form?

Relevant Equations

Newton's 3rd Law

I learned that standing waves form when the incident wave's frequency is equivalent to the reflected wave's frequency. But I also learned that according to Newton's 3rd law, when a wave hits a boundary, the reflected wave continues to travel in the opposite direction but has equal frequency and amplitude as the incident wave. But if this is the case, wouldn't the incident and reflected waves cancel each other out by destructive interference?

 

[haruspex]

To cancel at all times and places, they would have to be traveling in the same direction.

equal frequency and amplitude

There is a third parameter, and it is important in wave reflection.