Question

what is the focal length of double concave lens kept in air with two spherical surfaces of radii R1 = 30cm and R2 = 60cm. Take refractive index of lens as n = 1.5?


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Answer 1

using lens markers formula

1/f = (n-1) [1/R1-1/R2] (where R1=-ve)

1/f = (1.5 -1) [1/-30-1/60]

1/f = (0.5) [3/60]= -0.5×1/20

f= -40cm

I hope it's help you bro☺

Answer 2

Refractive index of a double concave lens (n) = 1.5

Radius of curvature of first face (R1) = –20cm Radius of curvature of second face (R2) = 60 cm Focal length of the given lens (f) = ?

len's makers formula : $\frac{1}{f} =n(n-1) ( \frac{1}{R1} - \frac{1}{R2} )$

∴ Focal length of the given lens, f = –30cm

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Calculation for substituting the values are given above.

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