According to Huygen’s principle ALL light propagation can be regarded as involving .
Simply put, the amplitude at some point further forward is the result of interference between waves coming from all points on a previous wave front. If there are no obstacles the front just continues undisturbed,
If howevere there is now some obstacle that blocks part of the previous wave front then some of the contributions are absent and the remaining intereference can become more or less constructive than without the obstacles leading to light and/or dark areas not present before.
