The wavelets along the front combine to make the straight-line wave front. But the slit allows only a few wavelets through. The wavelets on either side of the slit are blocked, so the wave front is no longer straight. The two waves combine to produce a single wave with a larger amplitude. The amplitude of the combined wave equals the sum of the amplitudes of the two original waves.
Scientists say that constructive interference takes place.
If the crest of one wave passes a point at the same time as the trough of the other, however, the waves combine to produce a wave with a smaller amplitude. The amplitude of the combined wave equals the difference between the amplitudes of the original waves. Destructive interference occurs. Philip L.-F. Liu, Sc.D., Professor of Environmental Engineering, School of Civil and Environmental Engineering, Cornell University. How to cite this article.