Volltext Seite (XML)
Fig. 10. A geo ¬ parallel surfaces, and we and that the reflection is least when the plane of the glass is perpendicular to the rays of light. This point is worthy of attention when considering the construction of a lens. We have only so far taken as an example of refraction the passage of a ray of light through a plate of glass with have found that its path, after such a passage, is parallel to its path before. We will next trace the path of a ray which strikes in any direction a glass prism of triangular section. Let A B C be a section of such a prism, and S a source of monochromatic light (such as the wick of a spirit lamp impregnated with common salt) sending a ray in the direc tion S 1). Then, when it reaches the surface of the glass A C at D, instead of travelling on towards E, it is bent towards the perpendicular P P', according to the law membered that this is not the only instance in which the ray, in passing from glass to air, is reflected. Whenever a ray strikes a surface common to two media, part of the light is reflected. This is easily proved experimentally. Hold a piece of glass so that the sun’s ray passes through it; it will be found that part is reflected, and that the more obliquely it is held, the stronger is the reflection, those already used), such as BCMN and B’CMN. Keep the screens on A C and A O', and the holes in them Fig. 12. we must suppose the light to travel in all directions, as is usually the case. Make two screens for the sides of the prisms nearest S, and pierce two small holes in any position, taking care that they are in the same vertical plane. Now, the only light which will pass through the prisms will be through the two holes, and if we follow the construction used in fig. 12, it will be seen that the light penetrating through one hole, D, will find a path towards H, and through the other towards H'. If, now, a piece of white paper be moved so that it reaches the point O, the two images of S will there coalesce, so that we shall have only one image, but twice as bright as the image formed by one hole. It is evident that we are only using a small portion of the prism, and that all except two small layers might be cut away without altering the result. Next, suppose we increase the number of the prisms by introducing two portions of other prisms (less pointed than any refraction, and will strike the surface PQinC. metrical construction will show that, in this case, the beam cannot emerge into the air; it is, therefore, totally reflected to D, following the law of ordinary reflection (see Lesson I.), strikes the surface, P R, of the glass perpen dicularly, and therefore emerges into the air along D E. It is on this principle that prisms of total reflection are made for obtaining reversed negatives. It should be re- it makes such an angle with the common surface that it can never get out. Let A O be the ray, and make the same construction as in fig 2, supposing the index of refrac tion of the glass to be 1}. Now, if 1, X A n be equal to or already enunciated, and travels, in the direction D G, and would travel on to G did it not meet with another surface, A B, common to glass and air. Here the ray, as it passes to a less dense medium (air), is bent away from he perpen- / R Fig. 9. greater than O d, which is radius of the circle, it is evident that A O can never emerge into the air. What becomes of it ? It can only be reflected, and it must be so totally. The angle that A O makes with O Q when the refracted ray grazes the surface is called the critical angle. Suppose we have a prism of glass (Fig. 1) which, in sec tion, has one angle, Q R P, a right angle, and the two sides, QRandP R, equal; if a ray, A B, strikes the glass prism perpendicularly, as shown, it will enter the glass without Fig. 11. dicular Q Q, and travels towards H. So, if the eye were placed any where in the line H F, the source of monochro matic light would appear to be at S'. Now, suppose we put two such prisms with their bases in contact, and trace the path of rays of light from some mint. In our last example we supposed that the ray of night was compelled to travel along S D (fig. 12). Here