# Difference between Concave and Convex Lens

## What is the difference between Concave and Convex Lens? What is a Concave Lens?

The lens which is thinner in the middle than at the edges is called a concave lens. A concave lens is a spherical lens whose two surfaces are concave. When we look at a concave lens from the front, it looks like a piece of transparent spherical glass having depression at the centre. We can feel the depression at the centre of a concave lens by touching it.

Double concave lens: If both the refracting surfaces of the lens are concave, then the lens is said to be a double concave lens or simply a concave lens. It is thinner in the middle and thicker at the edges.

### What is a Convex lens?

The lens which is thicker in the middle than at the edges is called a concave lens. A concave lens is a spherical lens whose two surfaces are convex. When we look at a convex lens from the front, it looks like a piece of transparent spherical glass. We can feel the bulge in the middle of the convex lens by touching it.

Double convex lens: If both the refracting of the lens is convex, then the lens is said to be a double convex lens or simply convex lens. It is thicker at middle and thinner at the edges.

### Image of Convex lens and Concave lens ## The important term associated with the concave and convex lens

1. Optical centre: The centre point ‘O’ on the principal axis, of the lens, through which incident ray of light passes undeviated with negligible displacement is called the optical centre of the lens.

1. Centre of curvature: The centre of the sphere, whose part is a spherical surface of the lens is called the centre of curvature. Since the lens have two spherical surfaces so two different centres of curvature C1 and C2 lie on either side of the surface.

1. The radius of curvature: The radius of the sphere from which the spherical surface of lens is made, is called the radius of curvature of the lens. So, each surface of a lens having separate radii of curvature R1 and R2

1. The principal axis of lens: A-line C1C2 joining the centre of curvatures C1 and C2 of the two spherical surfaces is the principal axis of the lens.

For the refraction by a spherical lens, we assume

• The lens is to be a thin having small thickness.
• The aperture of the thin lens is small, much less than its radius of curvature.

### The principal focus of the convex lens

Suppose a parallel beam of light is incident on one of the spherical surfaces of a convex lens, the rays after refraction through it, meet at a fixed point on the principal axis, this point is known as the principal focus of the convex lens.

### The principal focus of the concave lens

Suppose a parallel beam of light is incident on either of the spherical surface of a concave lens, after refraction through it, they appear to come from a fixed point on the principal axis. This point is known as the principal focus of a concave lens.

### The focal length of the lens

The distance of the principal focus from the optical centre of the spherical lens is called the focal lens of the lens. It is denoted by ‘f’. If the lens is surrounded by the same medium having equal refractive indices and an equal radius of curvature, then the two principal focal lengths of the lens will be equal.

The focal length of the lens depends upon the following factor:

• The radius of curvature of both the refracting surfaces.
• Refractive index of the material of the lens.

## Difference between concave lens and a convex lens in tabular form

Let’s start the explanations about the difference between the concave and convex lens in the table:

 Convex Lens Concave Lens A convex lens is a converging lens A concave lens is a diverging lens Thicker at centre Thinner in the middle Thinner at edges Thicker at edges Both refractive surfaces are convex Both refractive surfaces are concave It is used in telescopes, cameras, projectors, microscopes, magnifying glasses. It is used in wide-angle spy-hole indoor and myopia eye defect correction.

## How to find Focal Length

To get a rough focal length of a convex lens, focus a distant object to a screen, using the convex lens. The distance between the lens and the screen is called the focal length.

## The important fact of the concave and convex lens

• A concave lens always forms a virtual, erect, and smaller image whatever be the position of the object.
• Convex lens forms erect image only when the object is in between the optical centre and focus, but it is virtual.

## Uses of Concave and Convex Lens

### Uses of the concave lens

• A concave lens is used in peepholes in the doors of hotel rooms.
• A concave lens is used in myopia eye defect correction.
• It is used in making spectacles.
• It is used in making certain kinds of the telescope.

### Uses of the convex lens

• Convex lenses are used in the camera and digital camera, an optical device used for producing a diminished, real and inverted image on the film.

• A convex lens is used as a magnifying glass that produces a magnified image of an object. For a convex lens to produce a virtual, erect, and magnified image, it is a must that the object is placed between the focus and optical centre of the lens.
• It is used in the microscope and telescope. Both of these devices use the objective lens and eyepiece which is a complex lens.
• A convex lens is used in hypermetropia eye defect correction.
• It is used in paper burning glass.
• It is used in palm reading lenses.

## Magnifying glass The word ‘magnify’ means to make something look bigger or larger than it actually is. A convex lens is also known as a magnifying glass. A magnifying glass is used to read the very small print of a book conveniently. It is also used in the science laboratory to observe the various body parts of small animals such as a cockroach or earthworm clearly. A watchmaker uses a magnifying glass to see the small parts inside a watch clearly. It is also used by palmists to see the details of the lines of our hands.

Read this article and solve the important question of this topic. It is being given to all questions for homework.

1. What are convex and concave lenses?

2. What are the uses of convex lenses and concave lens?

3. What are two examples of a concave lens?

4. What are the five uses of the convex lens?

I hope you enjoy this post Difference between Concave and Convex Lens. Like this post and comment below for other topics. My Name is Mukesh Kumar. I am a Teacher, Blogger, Educational Content Writer, and Founder of CBSE Digital Education.