Normal sea waves close to the shore behave exactly like tsunami waves. Both a 'wind driven wave approaching the beach' and a 'tsunami wave during its entire journey' are shallow water waves meaning their wave length is longer than the water depth and their speed is proportional to the depth...
The way in which the Nazerre wave is created is explained in https://www.surfertoday.com/surfing/the-mechanics-of-the-nazare-canyon-wave where the critical importance of the underwater canyon is discussed, as is how the sea floor profile causes the wave to refract. Other factors also contribute...
It is almost certainly because the fan casing is rotating about a vertical axis causing the wires on the front to interact differently with the wires at the back depending on the angle.
The back wires sometimes align side-by-side with the front wires, blocking 'two wire widths of light' and...
You see the same Moiré fringe effect when driving and you approach a bridge with handrails held by vertical rods (say, 1 inch in diameter, 6 inches apart) crossing the road.
The near side rods and the far side rods "bunch together" and you see horizontally moving bands of brightness and darkness.
Thank you for the clarification though I think you are off topic.
The original post said "double convex" and "double concave" and asked "What exactly is a negative concave lens?" I was answering this limited case - what is a negative double concave lens? My answer is then correct.
The...
The statement is a bit confusing as negative and concave effectively mean the same thing.
A concave lens has a negative focal length so calling it a "negative concave" lens is somewhat tautological.
A convex lens has a positive focal length.
When you use the formula 1/u + 1/v = 1/f, f will be...