Which produces a coherent source?

Hint: The coherent source is a source that produces waves having constant phase difference and a single frequency. There are different methods of producing the coherent source, they are Young’s double slit arrangement, Lloyd’s single mirror method, Newton’s rings and Michelson’s interferometer.

Complete answer:
The characteristics of the coherent source are: the waves have a constant phase difference and a single frequency.
The methods of producing coherent sources: Division of the wave front and the Division of the amplitude.
Division of wave front: The wave front is divided into 2 or more parts with the help of lenses, prism and mirrors. The common methods are Young’s double slit arrangement and Lloyd’s single mirror method.
Division of amplitude: The amplitude of the incoming beam is divided into 2 or more parts by the partial reflection with the help of lenses, prism and mirrors. The common methods are Newton’s rings and Michelson’s interferometer.

The laser light and the sound waves produce coherent waves (the waves with the same frequency and the phase) so are the coherent sources.
\[\therefore \]The laser light and the sound waves are the coherent sources.
Additional information: The method of producing coherent source: Old method: The light from the monochromatic source is allowed to pass through a narrow slit followed by the two narrow slits present on the screen. The waves coming out of the second round of narrow slits will always be in phase, as the waves originate from the same wave front.New method: The laser is used as a coherent source.

Note:
The coherent source produces in-phase (constant) and the stable interference (single frequency) of light. An example of the coherent source is laser light. The incoherent source produces out of phase (variable) and unstable interference (multiple frequencies) of light. An example of an incoherent source is sunlight.