1.

A biconvex lens of focal length 20 cm is made of glass of refreactive index (3)/(2), when placed completely in water (n_(w)=(4)/(3)), its focal length will be

Answer»

20 cm
80 cm
15 cm
22.5 cm

Solution :`(1)/(20)=((3)/(2)-1)((1)/(R_(1))-(1)/(R_(2))) and (1)/(f_(w))=((3//2)/(4//3)-1)((1)/(R_(1))-(1)/(R_(2)))=((9)/(8)-1)((1)/(R_(1))-(1)/(R_(2)))`
`RARR""f_(w)=4xx20=80cm`


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