Biconvex Lens: Definition, Properties, Uses & Applications

Biconvex Lens is a lens that consists of two convex lens surfaces placed at the back of each other in a spherical form that has the same radius of curvature. It is also called a convex-convex lens or a plano-convex lens. Biconvex lenses have many uses in real life like magnifying or condensing lenses and magnifiers. In this lens, a parallel beam of light passes through the lens and then converges on the focus or a spot behind the lens. Therefore, the biconvex lens is a positive or converging lens. The biconvex lens formula is given by,

\({1 \over f}= ({\mu_2 \over \mu_1} - 1)({1 \over R_1} - {1 \over R_2} )\)

Key Terms: Biconvex Lens, Plano-convex lens, Optical Instruments, Focal Length, Refraction of Light, Real Image, Convex Lens

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What is Biconvex Lens?

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Biconvex lens consists of two convex surfaces in spherical form. They are used in optical instruments for focusing or controlling laser beams, quality of the image, etc. A parallel beam of light goes through the lens and converges into a focus in biconvex lenses.

• The distance from the lens to the focus behind the lens is known as the focal length of the lens. 
• Images are formed by Biconvex Lens due to the refraction of light.
• Refraction of light refers to the change in the direction of light when the light passes from one medium to another.
• A Rainbow is an example of the Refraction of Light.
• Therefore, the image formed by biconvex lenses is a real image. 

• There is a curvature on both sides of the lens and because of this, there are two focuses and two centers on both sides.
• There is a line known as the principal axis that is drawn in the middle of the lens.
• They are symmetrical lenses. Both the convex lenses in biconvex lenses or plano-convex lenses have the same radius of curvature.
• The center of the spherical convex lens is known as the optical center. When the two spheres intersect in a way that their optical centers coincide, a biconvex lens is formed.

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Biconvex Lens Formula

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The derivation of the Biconvex Lens formula is given below:

We know that, 

\({1 \over f} = {1 \over u} - {1 \over v} \)

The general equation for the refraction on the spherical surface is as given below:

\({\mu_2 \over v} - {\mu_2 \over u}= {\mu_2 - \mu_1 \over R} \)

The equation for the light ray coming from the medium μ₂  to the medium μ₁ becomes:

\({\mu_2 \over v} - {\mu_1 \over v_2}= {\mu_2 - \mu_1 \over R_2}{\mu_2 - \mu_1 \over R_1} \)

From the above equations, we get

\({1 \over v} - {1 \over u}= ({\mu_2 \over \mu_1} - 1)({1 \over R_1} - {1 \over R_2} )\)

As the light rays emerge from the object that is placed at infinity, so u = ∞ and v = f

So, we get the biconvex lens formula as:

• R₁ = radius of curvature of lens 1
• R₂ = radius of curvature of lens 2

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Properties of Biconvex Lens 

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The properties of the biconvex lens are as follows:

• The focal length of a biconvex lens is always positive.
• The focal length of the biconvex lens is short in length.
• Biconvex Lens can easily converge the incident light.
• Real and Virtual images can be formed on a biconvex lens.
• It reduces the spherical aberration and can cancel the distortion and the coma with a conjugate ratio corresponding to unity.
• The biconvex lens is symmetrical in form as they have equal radii on both sides of the lens. 
• They are usually recommended for virtual imaging in the case of real objects, and for a positive conjugate ratio between 0.2 to 5.

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Uses and Applications of Biconvex Lenses

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The uses of the biconvex lens are as follows:

• Biconvex Lens is used as a burning glass.
• A Human has a biconvex lens in their eye which is used to produce virtual images.
• It is a positive lens and is used for converging beams.
• Biconvex lens has many uses in real life like magnifying or condensing lenses and magnifiers.
• It is used in popular imaging instruments like microscopes, telescopes, projectors, and binoculars.

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Previous Year Questions

1. Two thin biconvex lenses have focal lengths … [KCET 2021]
2. A slit of width 'a' is illuminated by the red light of wavelength … [KCET 2021]
3. Which of the statements are correct with reference to single slit diffraction … [KCET 2021]
4. In Young's double-slit experiment a monochromatic source … [KCET 2021]
5. A double convex lens has focal length 25 cm … [NEET 2019]
6. A point object is placed at a distance of 20 cm from a thin planoconvex … [JEE Advanced 2006]
7. The graph between object distance u and image distance … [JEE Advanced 2006]
8. A biconvex lens of focal length f forms a circular image of radius r … [JEE Advanced]
9. A biconvex lens has a radius of curvature of magnitude … [NEET 2011]
10. Assume that light of wavelength 600nm is coming from a star … [NEET 2020]
11. An object is placed at a distance of 40 cm … [NEET 2018]
12. An equiconvex lens has power P … [NEET 2019]
13. You are asked to design a shaving mirror assuming … [JEE Mains 2015]
14. What is the position and nature of image formed by lens combination … [JEE Mains 2019]
15. Light is incident from a medium into air at two possible angles … [JEE Mains 2013]
16. Diameter of a plano-convex lens is … [JEE Mains 2013]
17. A convergent doublet of separated lenses … [JEE Mains 2018]
18. A convex lens is put 10cm from a light source … [JEE Mains 2019]

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Things to Remember

• Biconvex lens consists of two convex surfaces in spherical form.
• It has a positive focal length.
• The focal length of biconvex lens is short.
• Biconvex lens have a high ability to converge the incident light.
• Biconvex lens can form real as well as virtual images.
• They can minimize spherical aberration. The lenses have a distortion value whose conjugate ratio corresponds to unity.

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