Coherent light must have the same phase as well as the same frequency. Monochromatic light only has to have the same frequency. A coherent source is always monochromatic but a monochromatic source may or may not be a coherent source.

Are monochromatic waves coherent?

Monochromatic light is coherent, if the waves of the monochromatic light oscillate in the same direction and have the same frequency and the same phase.

Why monochromatic light is coherent?

Laser light is a source of coherent monochromatic light. Coherence – two waves are coherent if the phase difference between them is constant. For this to be the case they must have the same frequency. Monochromatic – means having only one wavelength of light present.