Two light sources are coherent. What must be true about the waves they emit?
They have a constant phase difference and the same frequency.
They have the same amplitude and are always in phase.
They travel at the same speed and have the same amplitude.
They have the same wavelength and travel in the exact same direction.
237 exam-style questions on CIE IGCSE Physics General properties of waves, covering General properties of waves. Each one has a worked solution and a mark scheme showing where the marks go.