# Convex Lens

## Convex Lens - Uses, Types and Magnification

What is Convex Lens?

The convex lens is a lens that converges rays of light that convey parallel to its principal axis (i.e. converges the incident rays towards the principal axis) which is relatively thick across the middle and thin at the lower and upper edges. The edges are curved outward rather than inward. It is used in front of the eye bends the incoming light sharply so the focal point shortens and the light focuses properly on the retina.

Generally, a convex lens can converge a beam of parallel rays to a point on the other side of the lens. This point is called a focus of the lens and its distance from the Optical Center of the beam is called the focal length. The radius of curvatures R1 and R2 of the spherical surfaces and the focal length of the lens ‘f’ are connected by an approximate equation.

For Mathematical Equation:

1/f = (n–1)(1/R1–1/R2)

Where, n is the refractive index .
R1 and R2 are the radii of curvature.
R1 is denoted as the surface very near to the light source.
R2 is denoted as the surface very far from the light source.
In the case of Double Convex Lens, the focal length is greater due to the presence of the second curved surface. Since many optical devices require longer focal lengths then Double Convex Lenses are more preferred.

Focal length: This is the distance between the center of a convex lens where parallel rays converge.

Principal Axis: A line passing through the center of the surface of a lens and through the centers of curvature of all segments of the lens.

For the convex lens, we can draw

• 1. A ray from the top of the object straight through the middle of the lens and its direction is not changed.

• 2. A ray from the top of the object parallel to the principal axis. It is refracted by the lens to pass through the focus

• ( Diagram of convex lens)

Formula:

1/f=1/v + 1/u

Where, f is focal length
v is denoted as the distance of the image from the optical center.
u is denoted as the distance of the object from the optical center

In convex lens, the focal length is positive.

Real image and Virtual image for convex lens

Real Image: A convex lens can be used to produce a real image, and this occurs if the object is located at a position of more than one focal length from the lens. It is projected in front of the lens and can be captured on a screen. It is used to the movie theater, projector etc.

Virtual Image: A convex lens will produce a virtual image if the object is located in front of the focal point. It is used in eyeglasses to give clear images.

Uses of the convex lens:

There are following uses:

• 1. It is used as Hypermetropedia i.e. long-sightedness.

• 2. It is used in microscopes, telescopes and magnifying glasses to subject all the light to a specific object.

• 3. It is used in camera lens because they focus light for a clear picture.

• 4. It is used in front of the eye bends the incoming light sharply so the focal point shortens and the light focuses properly on the retina.

• Why convex lens is called a converging lens?

A convex lens is called a converging lens because it converges a parallel beam of light on a point called the principal focus.

Magnification of convex lens:

It is a ratio between the image height and object height. A magnification of 2 indicates the image is twice the size of the object and a magnification of 1 indicates an image size being the same as the object size. If the magnification is positive, then the image is upright compared to the object (virtual image). If magnification is negative then the image is inverted as compared to the object (real image).

Types of convex lens:

1.Plano-convex lens: It is curved outwards from one side and the other side plane. It is positive focal length elements that have one spherical surface and one flat surface. These lenses are designed for infinite parallel light use in non-critical applications. These optical lenses are for all-purpose focusing elements. It is used to pharmaceutical, defense, robots etc.

2. Double convex lens: It is curved outwards from both the side. It is also known as the Biconvex lens or just convex. They have a shorter focal length than Plano-convex lenses of equal diameter and surface radius. So many optical devices require longer focal lengths. Hence, the double convex lenses are more preferred. It is used to the projector, monocular, Telescope, cameras etc. It produced the virtual image for the human eye and the real image for photography, an optical sensor and also used in burning glass

3. Concave -convex lens: It is curved inwards from one side and outwards from one side. It can be used to balance out the spherical aberrations caused by other lenses. It is used to controlling the laser beam. It is a combination of a lens with one convex lens and one concave lens side that is concave-convex lens or meniscus.

The functions of the convex lens:

1. When the object is at infinity then convex lens forms the image at focus which is real and inverted.

2. When the object is beyond the Imaginary point then an image is formed between the Focal point and an imaginary point which is real, inverted and diminished.

3. When the object is an imaginary point, then an image is formed at an imaginary point which is the real, inverted and of the same size.

4. When the object is between the Focal point and the imaginary point then an image is formed beyond an imaginary point which is real, inverted and magnified.

5. When the object is at Focal point then an image is at infinity which is real inverted and magnified.

6. When the object is between the Focal point and the Centre of curvature then the image is formed beyond the imaginary point and behind the object which is virtual and magnified.

Example: A 6-cm high object is placed 6 cm from an 18-cm focal length. Find out the image distance, the magnification of the image, the image height and the properties of the image.

Solution:

The focal length (f) = 18cm

The focal length is positive that is a convex lens. Then the focal point is real or the rays pass through the point.

The object height (ho) = 6 cm

The object distance (do) = 6 cm

Formation of an image by the convex lens:

The image distance (di):

1/di = 1/f – 1/do = 1/18 – 1/6 = 1/18 – 3/18 = -2/18

di = -18/2 = -9 cm

The negative sign denoted as the image is virtual or the rays do not pass through the image.

The magnification of image (m):

m = – di / do = -(-9)/6= 9/6 = 1.5

The positive sign denotes upright image.

The image height (hi):

m = hi / ho

hi = m ho = (1.5)6=9cm

The positive sign denotes that the image is upright.

The properties of the image:

1. It is virtual.

2. It is upright.

3. The image is greater than the object.

4. The image distance is greater than the object distance.