Two waves that are the same shape and 180 degrees out of phase with each other will cancel each other out as one's trough aligns with the other's crest.

MSN: Franken-Physics: Atoms split in two �� and put back together

They discovered that by shifting the "phase" of light so that the peaks of one wave line up with the valleys of an adjacent one, the two cancel each other, creating "shadows" between waves that are smaller than the waves themselves(see diagram, p. 141).

FORBES: Trick Of The Light

These five areas two Democratic, one Republican, two swing cancel each other out.

ECONOMIST: Swing states

They discovered that, by shifting the "phase" of light so that the peaks of one wave line up with the valleys of an adjacent wave, the two waves cancel each other out, creating "shadows" between waves that are smaller than the waves themselves(see chart).

FORBES: Trick of the Light