345. =Refraction of Light.=--When light passes from one medium into another it is bent from its course. This may be illustrated by Fig. 232, in which A B C D is a box, into which a candle, E, is shining. The candle is so placed that the shadow of the side A C falls at D. But let the box be filled with water, and now the shadow is removed to _d_, as if the candle were at _e_. This is because the rays of light from the candle, in passing from the air into the water, are bent or refracted so as to take a different direction. Here we have light passing from a rarer into a denser medium. Let us see now how it is when light passes from a denser medium into a rarer. This can be illustrated on Fig. 233. Let the vessel, A B C D, be empty, and let a coin be placed at O. Let the eye, E, be in such a position that a straight line, O G E, from the coin to the eye would strike the side of the vessel a little below the edge, or, in other words, that the edge of the vessel would prevent the eye from seeing it. If now, keeping the eye in this position, water be poured in up to a certain level, say F G, the coin comes into view. This is because light coming from the coin to L is bent into another direction, L E, and the coin therefore appears to the eye to be at K. In this case the refraction is _from_ the perpendicular, P Q, let down through the point L, where the light emerges from the denser into the rarer medium. But When light passes from a rarer into a denser medium the refraction is reversed--it is _toward_ the perpendicular. It is from this refraction of light that a stick partly immersed in water appears to the eye to be broken just at the surface of the water. [Illustration: Fig. 234.]