AIPMT | 2015Geometrical OpticsHard

Question

Two identical thin plano-convex glass lenses (refractive index 1.5) each having radius of curvature of 20 cm are placed with their convex surfaces in contact at the centre. The intervening space is filled with oil of   refractive index 1.7. The focal length of the combination is :-

Options

A.-25 cm
B.-50 cm
C.50 cm
D.-20 cm

Solution


From lens maker′s formula

we have          
                       
&                    
Now                
⇒                    
⇒                      f = − 50 cm
Hence option (2)

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