Question

Double convex lens made of glass is 20 cm what will be its new focal length and place in the middle refractive index 1.25

Answer 1

Let refractive index (M)=1.5 from air to glass

and from air to medium is = 1.25

Therefore from medium to glass

= 1.5/1.25

=1.2

1/f= (M-1)(1/R’+1/R”)

1/20 =(1.5- 1)(1R’+1/R”) and

1/f = (1.2–1)(1/R'-1/R”). dividing

f/20 = (1.5–1)/(1.2–1)

f= 20*0.5/0.2

= 10/0.2

= 50 cm.