1. A laser emits a beam of light. Light propagates as a wave. The wavefronts are flat, which means that all light propagates with the same phase.

2. An opaque object, e.g. an eggshell, contains tiny grains that scatter light and destroy the wavefront.

3. The little light that is transmitted is diffuse and seems to contain no information. Waves scattered from different points of the opaque object interfere. At points where the different waves are in phase, we see bright spots. When the waves cancel each other the screen remains dark.

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