Question

7) A double convex lens has two surfaces of equal radii R=30cm and refractive index n= 1.5. find the focal length [f] in cmRequired to answer. Single line text.

(1 Point)

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Answer 1

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It is given that :-

• Radius of curvature of lens = 30cm for both lenses.
• Refractive Index = 1.5
• Focal length = ?

We know that according to lens maker's formula :

$\frac{1}{f} = (\mu - 1) \times \frac{1}{r_1} + \frac{1}{r_2}$

After inputting the known values we get :-

$\mapsto \frac{1}{f} = (1.5 - 1) \times \frac{1}{30} + \frac{1}{30}$

$\mapsto \: \frac{1}{f} = 0.5 \times \frac{1}{15}$

$\mapsto \: \frac{1}{f} = \frac{0.5}{15}$

$\mapsto \: 15 = 0.5f$

$\mapsto \: f = \frac{15}{0.5} = \frac{150}{5}$

$\mapsto \: f = \cancel \frac{150}{5}$

$\mapsto \: f = 30cm$

Therefore the focal length of given lens is 30cm.

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