Question

A convex lens of refractive index 1.5 has a focal length of 20 cm in air. Calculate the change is its focal length when immersed in water of refractive index 4 3 .

Answer 1

According lens makers formula,

1/f=(u-1)(1/R1 -1/R2)

1/R1 - 1/R2 = 1/f(u-1)

=2/f

When lens is dipped in medium of refractive index 4/3 (i.e, 1.33)

1/f' = 1.5/1.33×2/f

f' = 3.91×f

= 3.91×20

=78.2cm

Change in focal length=78.2-20

=58.2cm

I hope this helps

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