=381. Total Reflection.=--It has been shown that when light passes from a _denser to a lighter_ medium, as from glass or water to air, that the beam is refracted _away_ from the perpendicular. This is illustrated in Fig. 373. The diagram represents the change in the course of a ray of light that passes through water to a surface with air above it. A ray striking perpendicularly passes through without refraction. Other rays show increasing refraction with increasing angle of incidence. For one ray the angle of refraction is so large that the refracted ray is parallel to the surface. When this condition is reached, the _angle of incidence_ is called the _critical angle_. Any increase in the angle of incidence causes all of the light to be reflected as is the beam _E_. This action is called _total reflection_, the course of the reflected ray being according to the law of reflection. _A right-angle prism_ (see Fig. 374) is often used where a mirror would ordinarily be employed, the total reflection occurring within the prism giving more satisfactory results than a mirror. See Art. 398 for a description of the Zeiss binocular field-glass for an example of this use of total reflection. [Illustration: FIG. 373.--An example of total reflection.] [Illustration: FIG. 374.--Total reflection in a right-angle prism.] The mirage (see Fig. 375) is an optical illusion by which distant objects, below the horizon, are sometimes plainly seen. This phenomenon is most frequently observed in hot, desert regions, when the air conditions are such that the lower strata near the ground are very much hotter than those above. These lower strata, having expanded the most, are less dense than the cooler ones above. Hence a ray of light traveling obliquely downward is refracted more and more until total reflection takes place. The images seen are inverted giving a representation of trees or other objects reflected on the surface of still water. The mirage is also frequently seen at sea, ships being observed, sometimes erect, sometimes inverted, apparently sailing in the clouds near the horizon. Over the Great Lakes, trees, boats, and towns on the opposite shore, sixty or seventy miles away, can sometimes be plainly seen, apparently but a few miles out. In this case the images are erect, the total reflection being from warm, still layers of air over colder layers near the water. [Illustration: FIG. 375.--Diagram of a mirage.] Important Topics (A) Refraction: cause, illustration, two principles. (B) Index of refraction, meaning. (C) Plates, prisms, lenses, action of each. (D) Total reflection, uses. Exercises 1. Compute the speed of light in water, the index of refraction being 1.33. 2. If one wished to shoot a fish under water, should he aim at the apparent location of the fish as viewed from the air? Explain, using a diagram.