Question

How do standing waves form?

Answer 1

Two progressive waves of the same type with the same frequency and amplitude meet and superpose to form standing waves.

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Usually a wave is reflected along the line of incidence to provide the two waves with the same properties.

In reality the progressive waves that form standing waves usually do not have identical properties, but they are very close. You can appreciate this from the relationship of intensity with distance travelled by the wave. If we are reflecting microwaves with a metal sheet to form a standing wave then the reflected waves will have travelled a larger distance before reaching the point at which interference happens. Its intensity will have decreased (according to `Iprop1/(d^2)`. And the reflected amplitude will have also decreased (according to `IpropA^2`).