CHAPTER VI REFRACTION OF LIGHT 42. Simple Experiments on Refraction. We have hitherto considered only the behaviour of light as it travels through a homogeneous medium (Arts. 2 and 4), in which case it is propagated in straight lines. But when a ray of light passes from one transparent medium into another, its direction Fig. 38.-REFRACTION OF BEAM OF LIGHT. is changed and it is said to be refracted. This bending or refraction of light may be illustrated by the following experi ments. EXPT. 21.—Procure a rectangular tin box (a biscuit-box). Mark a scale of inches or centimetres along the bottom of it, Take the box into a dark or lay a metal scale on the bottom. room and allow a beam of parallel light to fall slantwise against the edge. (It is best to use a strong source of lightsun-light or lime-light.) The side of the box throws a shadow C (Fig. 38), which is in a line with the direction of the incident beam AB. Note the point C where the edge of this shadow falls on the bottom. Now (without altering the position of anything) fill the box with water. The edge of the shadow moves to D, nearer the vertical side BN of the box. Clearly the light is refracted or Note the direction in which bent on entering the water. it is refracted. N'N is the normal at the point of incidence. On passing from air into water light is refracted towards the normal. EXPT. 22.- -Put a coin on the bottom of an empty basin (c, Fig. 39). Place your eye in such a position (E) that you just cannot see it-the coin being hidden by the side of the basin. If water is now poured into the basin (the eye still remaining at E), the coin becomes visible and appears to rise as the level of the water rises. This would clearly be impossible without some bending of the rays of light which proceed from the coin. A ray such as cs (which would otherwise not reach your eye) is refracted downwards along sE on leaving the water and thus enters your eye. The eye takes no notice of the refraction it simply sees the coin at d' along Es produced. : On pass Notice the direction in which the light is refracted. ing from water into air light is refracted away from the normal. EXPT. 23.-Plunge a stick or pencil slantwise into water. It looks as though it were bent just at the surface of the water, and the part immersed appears shortened and elevated (Fig. 40). This is best seen by placing the eye at one side: if you look along the stick, or if it is held upright in the water, it simply appears shortened and not bent. Fig. 40.-APPEARANCE OF STICK IN WATER. 43. Laws of Refraction.-Let RI (Fig. 41) represent a ray of light in air incident obliquely at I upon the surface of another medium, e.g. still water. Draw the normal at I to the refracting surface. The angle between the normal and the incident ray RI is called the angle of incidence. We know that on entering the water the ray of light is bent in some such direction as IS-i.e. towards the normal. The angle SIP between the normal and the refracted ray IS is called the angle of refraction. Clearly the angle of refraction is less than the angle of incidence. Produce RI to R'. The angle R'IS measures the amount by which the ray is bent or deviated out of its path: it is called the angle of deviation. Whenever a ray of light in passing from one medium to another is bent towards the normal, the second medium is said to be 'optically denser ' than the first. Thus water and glass are both optically denser 1 than air. With I as centre describe a circle. From the point where this circle cuts the incident ray draw a perpendicular to the normal: this is clearly equal in length to the perpendicular R'P' drawn from the point R' where the circle cuts the incident ray produced. Again, from the point where the circle cuts the refracted ray draw a perpendicular to the normal. The ratio of these perpendiculars has a constant value for any given pair of media (air and water, air and glass, etc.) We may now state the laws of refraction in the following form: I. The refracted ray lies in the plane containing the incident ray and the normal, and on the opposite side of the normal. II. If points equidistant from the point of incidence be taken on the incident and refracted rays, and if from these points perpendiculars be drawn to the normal, the ratio of these perpendiculars is constant for any given pair of media. 44. This constant ratio is called the index of refraction for the two media, and is usually denoted by the letter μ. Its value for air and water is about 4: for air and glass about 2 3 ; but it may be greater than this according to the composition of the glass. The following are approximate values of the indices of 1 This use of the term 'dense' must not be confused with the usual meaning of density as defined on pp. 24, 25. There is no necessary connection between the two. Thus turpentine is lighter than water but is optically denser: it floats upon the surface of water, but a ray of light in passing from water into turpentine is deviated towards the normal. |