Question

A double convex lens lens has two surfaces of equal radii R and refractive index =15 . find the focal length

Answer 1

Using lens makers formula,           1/f=(n-1)[(1/R1)-(1/R2)] Here n=15, R1= R and R2= -R Therefore,           1/f=(15-1)[(1/R)-(1/-R)]    =>.  1/f= 14×[2/R]    =>.  1/f= 28/R    =>.  f = R/28

Answer 2

Using lens makers formula,           1/f=(n-1)[(1/R1)-(1/R2)] Here n=15, R1= R and R2= -R Therefore,           1/f=(15-1)[(1/R)-(1/-R)]    =>.  1/f= 14×[2/R]    =>.  1/f= 28/R    =>.  f = R/28