This is a phased array: How can you focus sound?
It can focus many waves of weak radio wave or light into a single focal point that's very strong.
Now imagine that we delay half of the emitters by a constant amount that is equal to 0.5 of the period of the waveform. Now wouldn't we have evoked a standing wave at the focal point that is twice the frequency (but half the amplitude) of the emitted waves?
Now we can repeat it by delegating the emitters into 10 groups, and have them emit in increments of 1/10 the period of the wave. Wouldn't that evoke a standing wave that oscillates at 10x the frequency (but 1/10th the amplitude) of the original?
If scaled to a very extreme level, shouldn't we be able to generate visible light just by combining a whole bunch of radio waves? If not, why not?