Wednesday, 24 July 2013

Refraction of soldiers through a field of wheat

The Vertebrate eye and Its Adaptive Radiation. G.L. Walls (1942) - HERE 

'This bending of light rays when they pass through boundary surfaces is called 'refraction'. Its basis may be best understood if we use an old favorite analogy for our light beam and our pair of optically different substances. 

Suppose a platoon of soldiers to be marching over bare ground towards the edge of a wheat-field, which is at an angle to their line of march. The ranks of soldiers now represent successive wave-fronts in a light beam, and their files represent the individual light rays in the beam. Obviously the soldiers cannot march as fast through the dense wheat as over open ground, so that the latter may represent air, and the wheat-field a piece of glass of higher optical density.

As the first soldiers in the front rank start into the wheat, they are slowed up, but those at the other end of the front rank are still able to march rapidly since they have not yet reached the wheat (a). Consequently the front rank is swung around as if hinged at one end, and by the time the whole of the rank is in the wheat, it has taken a new direction of march which is of course followed by each rank in the whole platoon (b). 

Upon emerging from the wheat-field on the other side (c), the process is reversed and the platoon's line of march becomes parallel to its original one, displaced laterally a distance which depends upon the width of the wheat-field and the difficulty of marching through it'.


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