You probably know from experience that some seats in certain concert halls are undesireable because the reflected waves from the walls muffle the sound levels. Difficult as it may be to believe, light also produces these patterns. You don't see them very often because they are subtle and require special circumstances. This is because the distance between the wave crests, known as wavelength and often denoted by the Greek letter lambda, is very small. Common water and sound waves have wavelengths that are huge compared to those of visible light.
Light reflects off numerous microsopic grooves and interferes.
Holographic images are also produced in this manner. Two diffracting light waves will interfere with one another and reinforce their wave amplitudes at the intersection of their crests. Succeeding waves will interfere with waves which preceded them and traveled through the other slit. In this way, a series of rays where constructive interference occurs will appear to emanate from the midpoint between the two slits . The fact that light diffracts allows us to measure the distance between the crests of the wave.