I have been doing some reading on how rainbows are formed. If you have time, do some google searches on rainbows, halos and glory. It is just fascinating. In the process, I’ve been clearing out my basics on optics. Some of the concepts, like snell’s laws of reflection and refraction reminded me of the school days when I had unconsciously stuffed in all these materials into my head without understanding its significance or application. Anyway, I read about the behaviour of this interesting species called Archer fish and how it finds its prey.
To give you a quick tutorial on optics, when light enters from a less dense media to a higher denser media, it undergoes reflection (light emanating outwards) and refraction (light that passes through the denser media). The refracted light has a different degree from the normal when compared to the incident ray. The point is, to a normal eye, because of this refraction thingy, particles on the denser media appear slightly shifted. Explains why a pencil half dipped in water appears broken at the interface.
What does archer fish do? It stays inside the water and squirts a jet stream of water on its prey which hangs onto a grass or some form of vegetation near the surface of the water and knocks it off. Once the prey falls into water, it quickly rushes to the place where the insect fell. Sometimes, the archer fish goes out of water, of upto 20-30 cm and grabs the prey by its jaw. The mystery here is that, for a fish inside the water, the insect should seem to be in a different place (due to refraction) and yet, the archer fish accurately streams the water to the target with greater accuracy. One solution that partly explains the situation is that, by snell’s law (ni*sin(theta-i) = nr*sin(theta-r)). So, when theta-i=0, theta-r=0, which means that when the light enters perpendicular to the media, it doesnt shift inside the denser medium. So, if the archer fish is perpendicularly beneath its prey, it can exactly pinpoint where the prey is. However, it has been studied that the archer fish is able to predict the prey correctly even when the prey is at an angle. It is just interesting to know how nature works in mysterious ways….