To prove the refraction of light in a different way, take an upright empty vessel into a dark room ; make a small hole in the window-shutter, so that a beam of light may fall upon the bottom at a (fig. 4) where you may make a mark. Then fill the bason with water, without moving it out of its place, and you will see that the ray, instead of falling upon a, Will fall at b. If a piece of looking-glass be laid in the bottom of the vessel, the light will be reflected from it, and will be observed to suffer the same refraction as in coming in; only in a contrary direc tion. If the water be made a little mud dy, by putting into it a few drops of milk, and if the room be filled with dust, the 'rays will be rendered much more visi ble. The same may be proved by ano ther experiment. Put a piece of money Into the bason when empty, and walk back till you have just lost sight of the Money, which will be hidden by the edge of the bason. Then pour water into the bason, and you will see the moneydistinct ly, though you look at it exactly from the same spot as before. See (fig. 2) where the piece of money at S will appear at L. Hence also the straight oar, when partly immersed in water, will appear bent,-as C 8, If the rays of light fall upon a piece of flat glass, they are refracted into a direction nearer to the perpendicular, as described above, while they pass through the glass; but after coming again into air, they are refracted as much in the contrary direction ; so that they move exactly parallel to what they did before entering the glass. But, on account of the thinness of the glass, this deviation is generally overlooked, and it is considered as passing directly through the glass.
If parallel rays, a b (fig. 1) fall upon a piano convex lens, c d, they will be so refracted, as to unite in a point, c, be hind it ; and this point is called the " principal focus," or the " focus of pa rallel ray s;" the distance of which from the middle of the glass, is called the " focal distance," which is equal to twice the radius of the sphere, of which the lens is a portion.
When parallel rays, as A B (fig. 5) fall upon a double convex lens, they will be refracted, so as to meet in a focus, whose distance is equal to the ra dius or semi-diameter of the sphere of the lens.
Ex. 1. Let the rays of the sun pass through a convex lens into a dark room, and fall upon a sheet of white paper at the distance of the principal focus from the lens. 2. The rays of a candle in a room from which all exter nal light is excluded, passing through a convex lens, will form an image on white paper.
But if a lens be more convex on one side than on the other, the rule for find ing the focal distance is this : as the sum of the semi-diameters of both convexities is to the semi-diameter of either, so is double the semi-diameter of the other to the distance of the focus ; or divide the double product of the radii by their sums, and the quotient will be the dis tance sought.
Since all the rays of the sun which pass through a convex glass are collected together in its focus, the force of all their heat is collected into that part ; and is in proportion to the common heat of the sun, as the area of glass is to the area of the focus. Hence we see the reason why a convex glass causes the sun's rays to burn after passing through it. See Bunx IN6 glass.
All those rays cross the middle ray in the focus f, and then diverge from It to the contrary sides, in the same manner as they converged in coming to it. If ano tiler glass, F G, of the same convexity as DE, be placed in the rays at the same distance from the focus, it will refract them so, as that, after going out of it, they will be all parallel, as b c; and go on in the same manner as they came to the first glass, D E, but on the contrary sides of the middle ray. The rays diverge from any radiant point, as from a principal focus; therefore, if a candle be placed at f, in the focus of the convex glass F G, the diverging rays in the space Ff G will be so refracted by the glass, that, after going out of it, they will become paral lel, as shewn in the space c b. If the candle be placed nearer the glass than its focal distance, the rays will diverge, after passing through the glass, more or less, as the candle is more or less distant from the focus.
If the candle be placed further from the glass than its focal distance, the rays will converge, after passing through the glass, and meet in a point, which will be more or less distant from the glass, as the candle is nearer to, or further from, its focus.; and where the rays meet, they will form an inverted image of the flame of the candle; which may be seen on a paper placed in the meeting of the rays.
Hence, if any object, A B C (fig. 6), be placed beyond the focus, F, of the convex glass, d e f, some of the rays which flow from every point of the ob ject, on the side next the glass, will fall upon it, and after passing through it, they will be converged into as many points on the opposite side of the glass, where the image of every point will be formed, and consequently the image of the whole object, which will be invert ed. Thus the rays, A d, A e, A f, flow ing from the point A, will converge in the space, d a f, and by meeting at a, will there form the image of the point A. The rays, B d, Be, Bf, flowing from the point, B, will be united at b, by the refraction of the glass, and will there form the image of the point, B. And the rays, C d, C e, C f, flowing from the point, C, will be united at c, where they will form the image of the point, C. A nd so of all the intermediate points between A and C.