Question

an equi convex lens whose focal length is air is 20cm is made up of material whose refraction index is 1.5. if one of its surface is shivered and then dipped in to a transparent liquid whose refractive index is 1.6.in the liq it behave like a which Lena​

Answer 1

The value of focal length of water is 78.125 cm

Explanation:

Given that:

• Focal length in air = 20 cm
• Refractive index of air-water n₁= 1.33
• Refractive index of air-glass n₂= 1.5

For focal length in air:

Using the formula of the lens

1 /f air =(n2 / n1 - 1 ) [1 / R1 − 1 / R2]

Put the value into the formula

 1 / 20 = (1 / 1.5 −1)( 1 / R1−1 / R2)

1 / 20  = 0.5( 1 / R1−1 / R2 ) ...(I)

We need to calculate the focal length in water  

Using formula of lens

1 / f (water)  = ( 1.5 / 1.33 −1) 1 / R1 - 1 / R2

1 / f water = 0.128 [1/ R1− 1 / R2] ....(II)

Divided equation (I) by (II)

f (water) = 0.5 / 0.128  

f (water) = 0.5 / 0.128  × 20

f (water) = 78.125 cm

Thus the value of focal length of water is 78.125 cm