Concave Convex Lenses

Introduction

Have you ever seen a magnifying glass?  That is an example of a spherical lens. But do you know what type of lens it is? Let us go through this tutorial on the concave-convex lenses (spherical lenses) to know about the components and structure of the spherical lenses.

Introduction: Spherical Lenses

You may have heard and already learned about the plane reflecting surfaces as well as spherical reflecting surfaces. The spherical reflecting surfaces are of two types and can be referred to as concave and convex mirrors, as per their structure.

That is to say, if a spherical reflecting surface is curved inside, it is called a concave mirror. On the other hand, a spherical reflecting surface that is bulging outwards is known as a convex mirror.

Similar to spherical mirrors, the spherical lenses have one or two spherical sides. However, a key difference between mirrors and lenses is that the mirrors reflect the light rays whereas the lens refracts the light rays.

In other words, mirrors are not transparent but lenses are transparent. The light rays can penetrate the surface and passes through the lenses by getting refracted.

You may wonder what is meant by light rays getting refracted. Let’s answer the question, what is the meaning of refraction?

What is Refraction?

Refraction is a phenomenon where a light beam passes through a transparent spherical surface and changes its path. That is to say, a light ray gets deviated from its original path and travels in a different path after passing through a spherical surface. This change in the path is called refraction.

The angle also changes when a beam of light gets deviated from its original path, or let’s say, refracted. The angle of incidence and angle of refraction, therefore, differs.

Types of Spherical Lenses

Before proceeding further to the types of spherical lenses, let’s understand the structure of the spherical lenses first.

The lenses are made up of two spherical surfaces or one spherical and one plane surface. Both the surfaces are transparent and allow the light to pass through them. Such spherical surfaces are known as lenses.

There are two types of spherical lenses. They are as follows:

Convex Lens: A lens having one or two spherical surfaces that are bulging outwards is called a convex lens.

Convex lenses are thinner at the edges and thicker at the middle part.

Concave Lens:  A lens having one or two spherical surfaces that are curved inwards is called a concave lens.

Concave lenses are thicker at the edges and thinner at the middle part.

Images Coming soon

Image 1: Convex and Concave Lens

Components of Concave Convex Lens (Spherical Lens)

Principal Axis: A straight line passing through the lens from the middle and connecting both the Centre of Curvatures (C) is called the principal axis.

This is an imaginary line on which various points such as Focus (F), Centre of Curvature (C), and Optical Centre (O) lie.

Centre of Curvature (C): The two spherical surfaces of the lens are a part of two different spheres. When the two imaginary spheres are drawn which include the spherical surfaces of the lens and the center of both the spheres are marked, it is known as the Centre of Curvature of the lens.

As there are two spherical surfaces, there are two Centres of Curvature (C) for a single lens. Both the Centre of Curvatures are labeled as $\mathrm{C_1}$ and $\mathrm{C_2}$.

Optical Centre: The center point of the lens is known as the optical center. The optical center is denoted by the letter ‘O’. It is similar to the Pole (P) of the spherical mirrors.

When a ray of light incidents on the optical center, it does not change its original path and just passes through the ‘O’ without any change in the angle of incidence.

Radius of Curvature (R): The distance between optical center ‘O’ and Centre of Curvature ‘C’ is called the radius of curvature. It is denoted by the letter ‘R’.

There are two spherical surfaces and two radii of curvatures. Each radius of curvature is labeled as $\mathrm{R_1}$ and $\mathrm{R_2}$.

Both the radius of curvatures are of the same distance. That is to say, both Centre of Curvatures (C) lies equidistant from the Optical Centre (O) of a lens.

Aperture: The diameter of the circular surface of the lens is known as the aperture. It is denoted by the letter ‘A’.

It is important to have a small aperture compared to the radius of curvature (R) of the lens.

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Image 2: Components of Spherical Lens

Focus (F): A point on the principal axis where all the light rays converge is called a Focus. This point is denoted by the letter (F).

The convex lens has a real focus whereas the concave lens possesses a virtual focal point.

Activity to find a Focus (F) of the lens

To understand the next component of the spherical lens, we will perform an activity.

Requirements: Convex Lens, Paper

Procedure

• Hold the convex lens under the Sun and put paper below the lens.

• Adjust the lens in such a way that a bright dot is visible on the paper.

• Hold it for some time and observe what happens.

Observation: The paper starts to burn after some time due to the heat of the Sun.

Result and Conclusion: From this activity, we can conclude that a convex lens can converge the light rays into a point. This point is called Focus (F).

Focal Length (f): The distance between Focus (F) and Optical Centre (O) is known as focal length. It is denoted by the letter ‘f’.

Images Coming soon

Image 3: Focus (F) and Focal Length (f) of Spherical Lens

Conclusion

In this tutorial, we learned about the structure, types, and components of spherical lenses (concave-convex lenses). The components of a spherical lens include

• Aperture (A)
• Principal axis
• Optical center (O)
• The radius of curvature (R)
• The center of curvature (C)
• Focus (F), and
• Focal length (f).

FAQs

Q1. What is the difference between concave and convex?

Ans: A concave lens has a spherical surface that is curved inside, on the other hand, a convex lens has a spherical surface that bulges outwards.

Q2. How do you remember concave and convex lenses?

Ans: The word concave has another word ‘cave’ inside it. The structure of the lens which looks like a cave, which is curved inside, can be remembered as a concave lens. Another type of lens which do not have such a spherical surface can be referred to as a convex lens.

Q3. Where is the convex lens used?

Ans: Convex lenses are used in spectacles to correct vision abnormalities.

Q4. Is magnifying glass a convex lens?

Ans: Yes. The magnifying glass is a convex lens. When an object is placed between Focus (F) and Optical Centre (O), it is magnified by the convex lens.

Q5. Which lens is used in the human eye?

Ans: The human eyes have a ‘biconvex’ lens. That is to say, the human eye lens is two spherical surfaces, both of them bulge outwards. This biconvex lens helps to converge the rays onto the retina which produces a clear image of an object even in the dark.