A standing wave is formed when_____________.

Hint: The superposition of two waves which have equal amplitude and equal frequency but they are moving in opposite directions after reflection are called standing waves. Standing wave is formed due to the interface. Standing waves are also called stationary waves.

Complete answer:
The standing wave is formed due to the interference between two waves that have equal amplitude and equal frequencies but they are travelling in opposite directions. It is usually achieved by the traveling wave and its reflection only because both of them will have the same frequency and same amplitude.
The interface of two waves takes place when they are superimposed so their energies are either added together or are cancelled out.
The point at which the absolute value of amplitude is minimum are called the nodes, and the point where absolute value of amplitude is maximum are called antinodes.
Standing waves mostly cause resonance, when standing waves occur inside a resonator due to the interface between waves resonating back and forth in the resonator's resonance frequency.
If the waves of equal amplitude and equal frequency move in the same direction, the interface of both the waves results in travelling waves.
The standing wave was first discovered by Michael Faraday on the surface of a liquid in a vibrating container in 1831.
Certain examples of standing waves are –
Acoustic resonance- it is usually observed in strings and columns of air.
X-rays-These are usually used in the medical field.
Faraday waves- it is observed in air-liquid interface in hydrodynamic instability.
Seismic waves- it is observed on the surface of earth.
Visible light – it is usually observed in optical media such as optical waveguides and optical cavities.
Mechanical waves- it is usually observed in solid mediums due to resonance

Note:
Standing wave is also called a stationary wave so don’t get confused between the two of them. A standing wave is the interface of two opposite waves; they should not be in the same direction and the amplitude and frequency must be the same for the two waves.