How does the dark and bright fringes form ?

[yuganes warman]

When there is a water wave in a ripple tank( not involving any interference experiment ) there will be a pattern on the bottom of the ripple tank , that is the the dark and bright fringes. How does the bright fringes and dark fringes form for only water waves experiment in ripple tanks ? It involves the convex or concave lense concept, please explain. Thanks

 

[crador]

If you look from below, a water wave looks like a bunch of long concave and convex lenses trailing each other. As in, from below, a crest looks like it will collect light into a point, and a trough looks like it should spread out incident light. So you will have some fringes from light being collected (bright) or dispersed (dark).

This is a very rough guess, and I'm no expert, but it seems intuitive. Hope it helps.

 

[sophiecentaur]

Yes. It's hard to be any more specific about this but the fact is that the eye is very sensitive to patterns so it is aware of some very slight variations in brightness due to this 'focussing' effect. I think it could go either way (i.e. brighter/darker or darker/brighter) depending on the actual depth of the water relating to the focal lengths of the curved surfaces.