During Newton's time, a competing theory held that light was a wave the same as sound or water waves. This theory had been proposed earlier by Christiaan Huygens. It could explain the blurring at the edge of shadows by diffraction of light waves, as well as assigning the different colors of light to differing wavelengths. But a definitive proof of light's wave nature would require many more years.

If you look at waves formed by dropping a stone into a pond, the look like circles spreading outward from the center. When these waves encounter an obstruction (like a log or rock), they are blocked by it. But, the waves coming around either side bend inwards as they go around it so that, with continuing distance, the "shadow" of the obstruction begins to lose its edges.

Further, if you look at two sets of waves generated side by side, they overlap as they move in an interesting way. Where the crests of waves meet, the resulting wave is taller. Where two troughs of waves meet, the resulting wave is much deeper. And when the crest of one wave meets the trough of another, the wave seems to be gone. This occurrence is known as interference between the two sets of waves.