When an undulatory phenomenon finds a small obstacle in its path it is able to go round it. This is how we are able to hear a conversation on the other side of a wall. In the same way, when wavefronts come to a small opening they propagate from that point in all directions.
These two behaviours constitute diffraction, such a characteristic property of undulatory phenomena that the undulatory nature of light was only accepted when it was proved that it presented diffraction. The opening in the visual serves to explain the concept.

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