480 

 Hence 



or 



Dr. C. V. Drysdale on the 

 = F 4- F ~ £ mn g ^ ne °P^ lca l centre, 



t\ — ?it?J ^ ne convergence o£ the light 

 D °~ F 2 d focussing at 0. 



Fig. 6. 



A 









< cL a 





Nodal points of a Two-lens Combination. 



Similarly D,=-S£^. 



Hence 



D '=D -F : 



F Q + F 2 -F F 2 ^ 

 F 2 d 

 the convergence of the light proceeding to N , 



and 



*,'==, 



F 2 ,Z 



F + F 2 -F F 2 d 



the distance AN f 



*ile 



dJ 



F ^___ I the distance BN 2 . 



2 ~ F + F 2 -F F 2 ^ 

 We have seen above that the emergent convergence at the 

 second lens F= F o + F 2-FoF 2 ^ ^ nQw th&t ^ cQn _ 



1-F ^ ' 



F' 



vsrgence at any distance <t/ 2 = t — ^ 



w 



But d 9 = 



Fid 



; .«. 1+F'rf a = 



F 1 + F 2 -F 1 F 2 ^' •* - • - "' 1 + F^' 

 and therefore convergence at Nodal point = 



F + F 2 -F F 2 ^ 



1-F ^ 



(1-F ^) = F + F 2 -F FA = F ; 



