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J. Perelman
"Entertaining physics". Book 1.
Chapter 8. Reflection and refraction of light

WHY AND HOW REFRACTS LIGHT?

The fact that when moving from one environment to another beam of light is refracted, to many it seems an odd whim of nature. It seems incomprehensible why the light is not retained in the new environment of its initial direction, and elects a broken path. Those who think that, probably, was pleased to learn that the beam of light undergoes essentially the same thing that happens with the marching column of soldiers crossing the border between soil, comfortable for walking, and soil uncomfortable. Here is what the John Herschel, the famous astronomer and physicist of the last century.

“Imagine a group of soldiers walking on land divided by a straight border on two bands, one of which is smooth, flat and easy to walk, the other irregular, difficult, so walking on it can not be accomplished so quickly. Assume moreover that the front of the squad is the angle with the boundary line between the two bands, so that the soldiers reach this limit are not all simultaneously, but sequentially, one after the other. Then every soldier, crossing the border, will find himself on the ground on which he can no longer move as fast as up to that time. He will not be able to stay on the same line with the rest of the line, still located on the best soil, and will from it to keep up with every second more and more. Because every soldier, reaching the border, experiencing the same difficulty in walking, if the soldiers will not violate the order will not be scattered and will continue to March right column, all that part of the convoy that crossed the border, will inevitably lag behind the rest and will make it so obtuse angle at the point of crossing the border. And because the need to go up, without interrupting the roads each other, make each soldier to walk straight ahead, at right angles to the new front, the path that it will be at the border crossing, will be, first, perpendicular to the new front, and secondly, to relate to the way, which would be passed in the absence of deceleration, as the new speed to the former”.

In a small form you can reproduce this visual similarity of the refraction of light on the table. Cover half of the table cloth and slightly tilting the table, get to slide thereon a pair of wheels, tightly planted on a common axis (e.g., broken child locomotive or other toys).


Experience explaining the refraction of light.

If the direction of movement of the wheels and the edge of the tablecloth form a right angle, the refraction path does not occur. You have in this case an illustration of the optical rules: beam perpendicular to the plane of the partition, not refracted. When the direction oblique to the edge of the tablecloth, the path of the wheels islamified on this edge, i.e. at the boundary between media with different speeds of movement in them. It is easy to see that during the transition from the side of the table, where the speed is more (uncovered portion), in the part where the speed is lower (cloth), direction (“ray”) is close to perpendicular to the fall. In the opposite case there is destruction of this perpendicular.

This can, among other things, gather important information, revealing the essence of the phenomenon under consideration, namely, that the refraction is caused by the difference of the speed of light in both environments. The greater the difference in speed, the greater the refraction; the so-called “refractive index”, a measure of the intensity of a break-rays, there is nothing like the ratio of these speeds. When you read that the refractive index at the transition from air into water is 4/3, you, however, know that light moves in the air is about 1.3 times faster than in water.

And in this regard is another instructive feature of the propagation of light. If in the case of reflection, the light beam follows the shortest path, in the case of refractive he chooses the quickest way: no other direction does not cause the beam so soon to the “destination”, as this islomania way.

Entertaining physics J. Perelman

 




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