An equi-convex lens of focal length 10 cm and refractive index (`mu`g=1.5) is placed in a liquid whose refractive index varies with time as ` mu ( t ) = 1.0 + ( 1 ) /( 10) t `. If the lens was placed in the liquid at t = 0, after what time will the lens act as concave lens of focal length 20 cm?

Options

A. 5 sec
B. 10 sec
C. 15 sec
D. 20 sec

Answer

B

Explanation

` ( f _ e ) / ( f _ a ) = (( ""_a mu _ g - 1 ) _ a mu _ t ) /( "" _ a mu _ g - ""a mu _ t ) rArr - ( 20 ) /(10) =((1.5 - 1 )_ a mu _ t ) /( 1.5 ) `
` rArr 3 = 1.5 ""_ a mu _ t rArr t = 10` sec.