REFRACTION 



273 



arrow liave a chance to pass into the camera, producing a well-deHned image. 

 That is, the pencils from every point of the image consist of parallel rays. 



In the pinhole camera the rays of light traverse an optical medium which is 

 hoiiiof/eiu'ous, i.e. it is identical in its properties at all parts. The rays would 

 take a somewhat different path if the optical medium were heterogeneous. 

 For example, the medium might be denser or rarer at one part. When the 

 ray came to pass through this part it would suffer refraction. 



(4) Refraction. — When a ray of light passes from a rare to a dense medium 

 (or vice versa) it undergoes refraction, i.e. it is bent towards (or away from) 

 the perpendicular to the surface at the point of incidence. This perpendicular 

 is called the normal. Snell's Law states that for any two media, the sine of 

 the angle of incidence bears a constant ratio to the sine of the angle of 

 refraction. 



Refractive Index. In Fig. 69 PO is the incident ray, OQ the refracted ray, 



Fig. 60. — Befraction of incident ray PO at interface AB. 



NOM the normal to the interface AB between the media (the upper being 



sin PON 



the less dense). Then t, — = constant. This constant is called the 



sin QOM 



refractive index, and is usually denoted by the letter jx. 



(5) Reflection. In addition to this refraction, a part of the incident light 

 is reflected. The amount reflected varies with (i.) the obliquity of incidence, 

 (ii.) the difference in refractive index. 



(6) Lenses. A lens is a portion of any transparent medium bounded by 

 surfaces that are parts of a spherical surface. The line joining the centres of 

 the two spheres which bound a lens is called the principal axis of the lens. 

 There are two main types of lenses, viz. convex and concave. The former 

 cause rays to converge to a point on the principal axis, whereas the concave 

 lens causes rays apparently to diverge from a point on the principal axis. 

 The point to which or from which the rays appear to converge is, i)i the case 

 of parallel rays, the principal focus of the lens. 



Focal Length. The distance between the principal focus and the lens is 



B, IS 



