# Diffraction of water wave vs diffraction of light wave

Why isn't there fringes in diffraction of water wave but bright and dark fringes in the diffraction of light wave?

## Answers and Replies

Do constructive and destructive interference occur also in diffraction of water waves by single slit?

Andy Resnick
Science Advisor
Education Advisor
In a sense, yes. Propogation of a wave from an aperture can be thought of as a collection of plane waves all interfering to produce the observed diffraction pattern (plane-wave decomposition).

Surface waves can be very complicated because they are intrinsically a 3-dimensional problem, and a nonlinear problem (see, for example, Lamb "Hydrodynamics", chapter 9), so using them as an analogy to electromagnetic waves requires caution.

Also, I want to ask about the diffraction of light from single slit.
In theory, the single slit is regarded to have many strips(point sources)
By pointing different angles, the point sources can pairing up to cause some places at a distance d apart dark fringe. However, from the theory, it is said that we only consider the angle from the point souces is the same and parallel wave rays occur. Owing to a path difference and the actual distance between between the rays is small, destructive interference occur at those dark fringe places. But, I want to ask when the angle is different by a little bit, another ray from a particular point souce may reach the place of dark fringe, which is initially explained by cancelling of parallel rays from different pairing sources, so will this another ray which reach the dark fringe affect the dark fringe?

revelant source:
http://www.math.ubc.ca/~cass/courses/m309-03a/m309-projects/krzak/index.html

Andy Resnick
Science Advisor
Education Advisor
I'm not sure what you are asking. The far-field diffraction pattern is at infinity, so it makes more sense to cast the problem in terms of angles rather than distances, but otherwise, I can't quite parse your post.

Let say a board is placed at distance d from the slit.
We should find alternative dark and bright fringes there.
Let's consider the first dark fringe.
From the theory, point sources at the slit can pair up. For each pair, their wavelets will cancel each other at the dark fringe. Therefoe if we consider all the pairs, there will still be dark fringe. Take the pair of the highest and the middle point source as an example. In theory, their wavelets can cancel each other at the dark fringe. However, the wavelets are spherical. For the area enclosed by the two parallel rays from that two sources, the ray at a different angle from one of the sources may cross this area. But now this ray cannot find anything to have destructive interference. So is this strange?