The result of adding two waves of the same frequency depends on the value of the phase of the wave at the point in which the waves are added.
Electromagnetic waves are subject to interference.
For two sources of electromagnetic waves to interfere The sources must have the same frequency and polarization. The sources must be coherent. The superposition principle must apply. In the double slit experiment, a single source is split in two, to generate two coherent sources. When the light from the two sources is projected on a screen, an interference pattern is observed.