# A biconvex lens has radii of curvature 20 cm each. If the refractive index of the material of the lens is 1.5, what is its focal length?

By BYJU'S Exam Prep

Updated on: September 25th, 2023

(a) 20 cm

(b) -10 cm

(c) 10 cm

(d) -20 cm

If the refractive index of the material of the lens is 1.5, its focal length is 20 cm. In a biconvex lens, the first surface’s center of curvature is on the positive side while the second surface is on the negative.

Thus, R1 = 20 cm and R2 = -20 cm

We have,

1/f = (μ – 1) (1/R1 – 1/R2)

Substituting the values we get

1/f = (1.5 – 1) (1/20 – 1/-20)

1/f = 1/20

f = 20 cm

### Properties of Biconvex Lens

• Positive focal length is stated to be a property of biconvex lenses.
• It is stated that their focal lengths are shorter.
• They have the capacity to produce both actual and virtual images due to their ability to converge incident light.
• With a conjugate ratio equal to unity, they can erase the coma and distortion in addition to minimising spherical aberration.

Applications of Biconvex Lens

• Biconvex lenses are employed as condensing or magnifying devices.
• They can also be used as magnifiers or objectives.
• Many imaging systems, including telescopes, monoculars, microscopes, binoculars, cameras, projectors, etc., use these.
• These are used to create a real image for photographic films or optical sensors or a virtual image for human eyes.
• Additionally, these lenses are utilised as burning glasses.
• These are employed in a variety of sectors, including those that utilise picture relays.

Summary:

## A biconvex lens has radii of curvature 20 cm each. If the refractive index of the material of the lens is 1.5, what is its focal length? (a) 20 cm (b) -10 cm (c) 10 cm (d) -20 cm

A biconvex lens has radii of curvature 20 cm each. If the refractive index of the material of the lens is 1.5, its focal length is 20 cm. Biconvex lenses are used as magnifying or condensing lenses.

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