Question

What becomes of the energy of light waves in destructive interference ?

Answer 1

do you learn about law of energy?
energy is neither created nor destroyed. just transfer one form to another or energy is redistributed.

of course, it is the same case. here we observed energy becomes less or disappear when monochromatic light in destructive interference.
here, the energy gets redistributed such that energy disappearing in the regions of destructive interference will appear in the regions of constructive interference.