Question

The radii of curvature of a double convex lens are 15 cm and 30 cm and its refractive index is 1.5 calculate the focal length of the lens.

Answer 1

we know the formula

1/f=R.I(-1)(1/R1-1/R2)

=1.5(-1)(1/15-1/30)  ,     =-1.5(1/30)

=      1/f=  -1/20 ,     f=-20

focal length=-20cm

Answer 2

Answer: 6.06cm

Explanation: we can use the lens maker formula to solve this question

The formula is given by

1/f = (n-1){(1/R1)-(1/R2)}

Here

R1= 15cm

R2= 30cm

Refractive index n=1.5

We've to find focal length

Thus, using the lens maker formula

1/f= (1.5-1) ((1/15)-(1/30))

= (0.5) (0.066-0.033)

= 0.5*0.033

=0.165

Hence f= 1/0.165

f= 6.06cm

The focal length is 6.06cm