A convex lens is a transparent optical device that converges light rays towards a focal point, commonly used in various applications such as glasses, microscopes, and cameras. It can create both real and virtual images depending on the object's position relative to the lens, with several types including plano-convex, double convex, and concave-convex lenses. The document also details the lens's functions, mathematical equations for focal length, and common uses in correcting vision and enhancing image clarity.

1. Convex Lens - A Complete Overview
A lens is a transparent material that concentrates or disperses light rays when it passes
through them by refraction. According to the shape and purpose of the lens, they are
classified into two types convex lens and concave lens. Let’s learn about convex lenses
in detail in this article.
Table of Content
● Convex lens definition
● Real Image and Virtual Image for Convex Lens
● Types of Lens
● Uses of the Convex lens
● Functions of Convex Lens
● Convex lens Example
● Frequently Asked Question (FAQs)
Convex Lens Definition

2. A Convex Lens merges the rays of light it transmits parallel to its principal axis (i.e.,
converges the incident rays towards the principal axis), which is relatively thick across
the center and are thin at the bottom and top edges. The edges are curved outwards
rather than inwards. It is generally used in front of the eye to sharply bend incident light
so that the focal length is shortened and the light is focused correctly on the retina.
In general, a convex lens can converge a beam of parallel rays to a point on the other
side of the lens. This point is defined as the focal point of the lens, and its distance from
the optical center of the ray is called the focal length. The radius of curvature R1 and R2
of the spherical surfaces and the focal length of the lens "f" are related by an
approximate equation.
Mathem​
atical Equation
1/f = (n-1) (1/R1 - 1/R2)
Where n is the refractive index.
R1 and R2 are radii of curvature.
R1 = surface very near to the light source.
R2 = surface very far from the light source.
For Double Convex Lens, the focal length is greater due to the presence of the second
curved surface. Thus, many optical devices require longer focal lengths and Double
Convex Lenses are preferred.
For the Co​
nvex Lens
A ray on the object from the top parallel to the principal axis. It is then refracted by the
lens to pass through the focus.
A ray on the object from the top straight through the center of the lens, and there is no
change in direction.

3. Formula
1/f = 1/v + 1/u
Where f is focal length.
● v = image distance from the center
● u = object distance from the center
The focal length is positive in the case of a convex lens.
Real Image and Virtual Image for Convex Lens
● Real Image - A convex lens is used to create a real image, which occurs when
the object has more than one focal length away from the lens. It is projected in
front of the lens and can be captured on screen and used in cinemas, projectors,
etc.
● Virtual image - A convex lens creates a virtual image if the object is placed in
front of the focus. It is used in glasses to provide a clear image.
Types of Lens
● Plano-convex Lens
It is curved outwards from one side to the other side. It has a positive focal length that
has one spherical surface and one flat surface. These lenses are designed to use

4. infinite parallel light in non-critical applications. These optical lenses are intended for
focusing elements. It is used in defense, robots, pharmaceuticals, etc.
● Double Convex Lens
It is bent outward on both sides. It is also known as a biconvex lens or just convex. They
have a shorter focal length than the Plano-convex lenses of the same diameter and
surface radius. Many optical devices require longer focal lengths. Therefore, double
convex lenses are preferable. It is used for projectors, monoculars, telescopes,
cameras, etc., producing a virtual image for the human eye and a real image for
photography, as an optical sensor, and in glass firing.
● Concave-convex Lens
It is bent inward on one side and outward on the other. It can be used to compensate for
spherical aberrations caused by different lenses. It is generally used to control the laser
beam. It is a mixture of both lenses with one convex lens and one side of the concave
lens, which is a concave-convex lens or meniscus.
Uses of the Convex lens
1. It is generally used as hypermetropia, i.e., to correct farsightedness.
2. It is used in microscopes, telescopes, and magnifiers to expose all the light to a
particular object.
3. It is also used in camera lenses because they focus light for a clear image.
4. It is also used in front of the eye to sharply bend incident light so that the focal
length is shortened and the light is properly focused on the retina.
5. It is also used in projectors, telescopes, optical microscopes, and even the
peepholes in our homes' doors.
Functions of Convex Lens
1. When the object is at infinity, so convex lens forms an image at the focus that is
real and inverted.
2. When the object is behind the imaginary point, an image is formed between the
focus and the imaginary point that is real, inverted, and diminished.
3. When the object is at an imaginary point, then the image is created at the
imaginary point that is real, inverted, and the same size.
4. When the object is between the focus and the imaginary point, an image is
formed behind the imaginary point that is real, inverted, and magnified.
5. When the object is in focus, then the image is at infinity, which is actually inverted
and magnified.

5. 6. When the object is between the focus and the center of curvature, then the
image is formed behind the imaginary point and behind the object, which is
virtual and magnified.
Convex lens Example
Q1. A 6 cm high object is placed at 6 cm from an 18-cm focal length. Determine
the image distance, the magnification of the image, the image height, and the
properties of the image.
Solution: focal length (f) = 18cm
A focal length is positive, that is, a convex lens. So, the focal point is real, or the rays
pass through the point.
object height (ho) = 6 cm
object distance (do) = 6 cm
Image formation by the Convex Lens:
Image Distance (di):
1/di = 1/f – 1/do
= 1/18 – 1/6
= 1/18 – 3/18
= -2/18
di = -18/2 = -9 cm
Therefore negative sign denoted the image is virtual or the rays do not pass through the
image.
Magnification of Image (m):
m = – di / do
= -(-9)/6= 9/6
= 1.5
A positive sign represents an upright image.

6. Image Height (hi):
m = hi / ho
hi = m ho
= (1.5)6
=9cm
A positive sign represents that the image is upright.
Properties of the Image:
● It is upright.
● It is virtual.
● The image distance is much greater than the object distance.
● The image is greater than the object.
Frequently Asked Questions (FAQs)
Q1. What are the three types of the convex lens?
Ans. There are three types of these lenses; they are the plano-convex lens, the double
convex lens, and the concave-convex lens.
Q2. What can a convex lens be used for?
Ans. It is used as a Hypermetropia, i.e., to correct far-sightedness. It is used in
telescopes, microscopes, and magnifying glasses to subject all the light to a specific
object. It is also used in camera lenses because they focus light for a clear picture.
Q3. What image is formed by a convex lens?
Ans. We can have diminished inverted images, small sizes inverted images, enlarged
inverted images, and enlarged erect images.
Q4. Which kind of image is not formed by a convex lens?
Ans. The virtual image is not formed by a convex lens.
Q5. Are convex lenses always real?

7. Ans. The convex lens always forms a real image. The convex lens always forms a real
image.
Q6. Why is a convex Lens called a converging Lens?
Ans. A convex lens is called a converging lens because it converges a parallel light
beam on the principal focal point.
Original source: https://www.pw.live/physics-articles/convex-lens