OPTICS. 



OPTICS is the science of light and vision. 



All visible bodies may be divided into two classes self-luminous and non-lu- 

 minous. The first comprise those bodies which shine by their own light, as the 

 stars, sun, flames, &c. Non-luminous bodies are those which have no power of 

 discharging light of themselves, but which throw back the light that falls upon 

 them from self-luminous bodies. Light emitted from a self-luminous body is 

 projected in straight lines in every possible direction, so that the luminous body 

 not only seems the general centre whence all the rays proceed, but every point 

 of it may be considered as a centre which radiates light in every direction (fig. 1). 

 A ray of light is a straight line of light projected from a luminous body, and a 

 pencil of rays is a collection of rays proceeding from any one point of a luminous 

 body (fig. 2). 



When rays of light meet with an opaque body through which they cannot pass, 

 they are stopped short in their course, and cause the opaque body to form a 

 shadow on the other side of it. If the luminous body (as A, fig. 3) be larger 

 than the opaque body (B), the shadow will gradually diminish in size till it ter- 

 minates in a point ; if smaller, the shadow will increase in size, as it is more 

 distant from the object that projects it (fig. 4). A number of lights in different 

 directions, .while they decrease the intensity of the shadows, increase their num- 

 oor 7 which corresponds witnthat of the lights (fig. 5). 



Reflection Of Light- When a ray of light falls perpendicularly on an 

 opaque body, it is reflected back in the same line towards the point whence it 

 proceeded ; if it falls obliquely, it is reflected obliquely, but in the opposite direc- 

 tion, the angle of reflection being equal to the angle of incidence (fig. 6). 



The principle of reflection may be illustrated by the case of a plane mirror or 

 looking glass (A B. fig. 7). The ray from the eye of the spectator at c will be 

 reflected in the same line C A ; but the ray D B, coming from the foot, in order 

 to be seen at the eye, must be reflected in the line B c, and therefore the foot 

 will appear behind the glass at E, along the line c B E. 



There are three kinds of mirrors used in optics ; the plane or flat, the convex, 

 and the concave. A convex mirror has the peculiar property of making the 

 reflected rays diverge ; and a concave mirror makes the rays converge. 



M N, fig. 1, represents a convex mirror formed of a portion of the exterior of a 

 sphere, whose centre is o. ABC are three parallel rays, the middle one of which 

 only is perpendicular to the centre of the mirror, and is reflected in the same 

 line ; the two others falling obliquely, will be reflected obliquely to G and H, the 

 dotted lines, P P, being perpendiculars which divide their angles of incidence and 

 reflection. By continuing the reflected rays G B and H E, backwards, they will 

 meet at F, their virtual focus behind the mirror. 



If A B, fig. 9, be an object placed before the convex mirror (M K), and lines 

 be drawn from its extreme points A B, to o, the centre of the sphere, a diminished 

 representation of the objects will be formed at the focus. 



Fig. 10 illustrates the property of a concave mirror. ABC are three 

 parallel rays, which, striking the mirror, are reflected, the middle ray in the same 

 line, and the two others converged so ^s to meet at the focus F, midway between 

 the surface of the mirror, and the centre of its concavity c. The dotted lines P P, 

 are perpendiculars. 



Images reflected from concave mirrors appear larger than the real objects, pro- 

 vided these objects are within the focus, as A B, fig. 11. 



A B, fig. 12, is an object placed at some distance in front of a concave mirror. 

 In this case, a small and inverted image of the object will be formed at D E. 

 When the object is placed at D E, a magnified representation of it will be 

 formed at A B. 



Concave mirrors are used as burning glasses (fig. 13). 



Refraction Of Light* Refraction is the effect which transparent mediums 

 produce on light on its passage through them. Opaque bodies reflect the rays j 

 transparent bodies transmit them ; but it is found that if a ray, in passing from 

 one medium into another of different density, fall obliquely, it is turned out of 

 its course, or refracted ; but if it fall perpendicularly it is not refracted, but pro- 

 ceeds through the new medium in its original direction. 



Let fig. 14, be a vessel half filled with water. If a /ay strike it in the 

 perpendicular direction, A B, it will be continued in the same line to c ; but if a 

 ray be admitted at D, it will strike the water obliquely at B, when it will become 

 subject to two opposite forces, the attraction of the water endeavouring to draw 



perpendicularly to c, and the projectile force of the 3fay inducing it to proceed 



