# Calculate the Radius of Curvature of an Equi-concave Lens of Refractive Index 1.5, When It is Kept in a Medium of Refractive Index 1.4, to Have a Power of -5d? - Physics

Sum

Calculate the radius of curvature of an equi-concave lens of refractive index 1.5, when it is kept in a medium of refractive index 1.4, to have a power of -5D?

#### Solution

Using lens maker formulae for given equal concave lens,

1/f = (n_2/n_1 - 1)(1/-R - 1/R)

⇒ 1/f = (n_2/n_1 - 1)(-2/R)

Where n2 and n1 are refractive index of the lens and medium, respectively where

n= 1.5 & n1 = 1.4

Power of lens -5D

Focal length f = 1/-5 xx 100 = -20cm

Putting all the values in equation 1,

⇒ 1/-20 = (1.5/1.4 - 1)(-2/R)

⇒ 1/20 = (0.1/1.4)(2/R)

⇒ R = 4/1.4 = 2.86 cm

Concept: Lenses
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