DIOPTRICS 217 



PJ? PO 



But in the similar AS FOR, VOS, ^ = , 



i po 

 and - - = /mby construction. 



Sin PON 



A = /x. Hence OQ is the refracted ray. 

 Sin QOM 



If PO is perpendicular to AB, sin PON =0. Hence sin QOM =O, 

 i.e. the ray is unrefracted in its passage. 



But, 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. 



When a source of light, e.g. a candle, is placed near a biconvex 

 lens, we see very clearly two images produced by reflection. 

 The first is formed by the anterior convex surface and is upright, 

 the second is formed at the posterior surface, which is concave 

 in respect of rays passing out of the lens into the atmosphere, 

 and is inverted. The size of the image decreases as the convexity 

 of the lens increases. Its brightness increases, the more obliquely 

 the rays from the candle strike the surface and also with increase 

 of the refractive index of the medium composing the lens. 



(2) Images formed by a Convex Lens, (a) When the object is 

 between oo and 2/ (/ being the focal length), the image lies 

 between -/ and - 2/, and is real, inverted and diminished. 



(b) When the object is between 2/and/, the image lies between 

 2/and oo , and is real, inverted and magnified. 



(c) When the object is between /and the optical centre of the 

 lens, the image lies between +00 and the optical centre of the 

 lens, and is virtual, erect and magnified. 



(3) Chromatic Aberration. When white light passes through a 

 lens its component rays are unequally refracted, the violet -being 



FIG. 48. Chromatic Aberration. 



brought to a focus nearer the lens than the red. If the image 

 is received on a screen midway between these points it will 

 appear to be surrounded by a red and violet halo. 



