Convex Lens: Definition, Ray Diagram, Focal Length, Image Formation, Types & Uses

A convex lens, also known as a converging lens, is a type of lens that curves outward like a sphere on both sides. It is thicker at the center and thinner at the edges. They cause light rays to converge or come together at a point, forming a focused image. They are commonly used in various optical instruments, including eyeglasses, magnifying glasses, telescopes, and microscopes. A convex lens is an important component in cameras, telescopes, microscopes, and other devices. Although its presence is universal in modern technology, its function and mechanics might not be widely understood.

What is a Convex Lens?

In most instances, an optical lens is composed of two spherical surfaces. This lens has thinner margins and a thicker centre than its edges. If those surfaces are curved outwards, the lens is referred to as a biconvex lens or just a convex lens. The image formed by a convex lens can be both real and virtual images based on the object's position. The erect image by convex lens is always virtual and diminished image of the object. A beam of light coming from the outside can be focused by these lenses to a place on the other side. The focal length of a convex lens is the distance from the lens’s centre to the focus. The focus is at this point. On the other hand, a Plano-convex lens is created when one of the surfaces is convex and the other is flat.

Fig-Convex Lens Ray Diagram

Image Formation by Convex Lens

• Object at infinity: A real and pointed picture is formed at the convex lens’s focal point when the incoming parallel rays are focused on the lens focal point by the convex lens.

• When an object is positioned outside the curvature’s centre, a parallel beam of light strikes the lens and, after refraction of light, passes through the focal point. Additionally, a light beam travels through the optical centre, creating an image between the focal point and the curvature’s centre.

• When an object is positioned in the centre of the curvature, a parallel beam of light strikes the lens and, after refraction, passes through the focal point. A genuine picture of the same size is created at another centre of the curvature as a result of a light beam passing through the optical centre as well.

• When an object is placed between the centre of curvature and the focal point, a parallel beam of light strikes the lens and, following refraction, passes through the focal point Additionally, a light beam travels through the optical centre, creating an actual image behind the centre of the curvature that is greater than the item.

• When an object is positioned at the focal point, a parallel light beam strikes the lens and, following refraction, passes through another focus point. Additionally, a light beam travels through the optical centre, creating a true image at infinity that is much larger in scale.

• When an object is positioned between the focal point and the optical centre, a parallel beam of light strikes the lens and, following refraction, passes through a different focus point. Since a light beam also travels through the optical centre, a larger virtual image is created behind the centre of curvature.

Focal Length of Convex Lens

The focal length is the separation between the lens’ optical centre and its focus. An optical system’s focal length, which is the inverse of the system’s optical power, indicates how strongly the system converges or diverges light. The refracted rays of light from the parallel beam converge on the opposite side of the convex lens. If the image is obtained at the lens’s focus, it will be actual, inverted, and very small. The focal length, f, is the distance between the optical center of the lens and the primary focus. Because the image created by the lens is actual, it can be obtained on the screen.

Magnification of Convex Lens

The height of an image divided by the height of an object is known as the magnifying power of a convex lens. Magnification is also expressed as . If the magnification is positive, the picture is vertical in relation to the object (virtual image). If the magnification is negative, the image is inverted in relation to the object (real image).

Also, learn the Difference Between Real Image and Virtual Image

Sign Convention for Convex Lens

The sign convention for a convex lens is typically as follows:

• Focal point (F) is positive if on the left side
• Object distance (u) is positive if the object is on the left side
• Image distance (v) is positive if the image is formed on the right side

Types of Convex lens

There are three types of convex llens,which are explained below.

• Plano Convex Lens
• Double Convex Lens
• Concave Convex Lens

Plano Convex Lens

An optical element called a Plano-convex lens is used to concentrate light into a single point. One side of the lens is described as being flat (a plane) in the “Plano” section, and the other is described as being bent outward in the “convex” portion. These lenses are made to be used in non-critical applications with unlimited parallel light. It is used in robots, defense, and pharmaceutical products.

Double Convex lens

It has an outward curvature on both sides. It is sometimes referred to as a convex lens or a biconvex lens. In comparison to Plano-convex lenses of the same surface radius and diameter, they have a shorter focal length.

Concave-convex lens

It has an outward curve from one side and an inward curve from one side. It can be used to correct other lenses’ spherical aberrations. It is applied to laser beam control. Concave-convex lens or meniscus is a mixture of both llenses,with one convex lens and one concave lens side.

Formula of Convex Lens

The relationship between the focal length of the lens, the object’s distance, and the image’s distance can be calculated using the following lens formula.

Where,

• f= focal length
• v = the image’s separation from the optical centre
• u = the separation between the object and the optical centre.

If the lens is convex, then the focal length will be positive (converging). The image created will be virtual and are on the same side of the lens as that of the object if the image distance is negative.

Difference Between Concave and Convex

The difference between convex and concave lenses is listed in the table given below:

CONCAVE LENS	CONVEX LENS
A concave lens is characterised as a lens that disperses the light beam.	A convex lens is one in which the rays reflected through the surface are dispersed in a parallel path using reflection of light.
A diverging lens is another name for a concave lens.	It is known as converging lens.
One curved surface characterises a concave lens. The surface has an inward curvature.	The two spherical surfaces of a convex lens, on the other hand, are one of which is bent outward.
Real focus	Virtual focus.
The magnification of a concave mirror is more, equal, or less than one.	The magnification of a convex mirror is less than one.
A concave lens is thin in the centre and has thick edges.	A convex lens has a thick centre and narrow edges.
For those with myopia, concave lenses are used.	For those with hypermetropia, a convex lens is employed.
A concave mirror has a negative focal length.	A convex mirror has a positive focus length.
A concave lens produces a small, virtual, and upright picture.	A convex lens can produce an image that is either small or huge in size. The resulting image is accurate and reversed.
In devices like flashlights, cameras, and lasers, concave lenses are employed.	Objects like magnifying glasses, microscopes, and other similar devices use convex lenses.

Uses of Convex Lens

Following are the uses of convex lens.

• Projectors: Projectors utilize convex lenses to enlarge images or videos onto screens, correcting the orientation of the projected image by inverting the film.
• Cameras: Convex lenses in cameras help focus and magnify images. Adjusting the position of these lenses changes the magnification and focal point, enhancing image clarity in devices like webcams and video cameras.
• Magnifying Glasses: These use a single convex lens to focus light at a focal point, magnifying objects when held at a distance shorter than the lens’s focal length.
• Eyeglasses: For individuals with hyperopia (far-sightedness), convex lenses are used in glasses to correct the focus of light onto the retina, improving near vision.
• Peepholes: Convex lenses in door peepholes magnify the view, allowing a broader and clearer view of the outside from a small aperture.
• Microscopes: In microscopy, convex lenses magnify small objects like bacteria, using multiple lenses to enhance the image significantly.
• Refracting Telescopes: These telescopes use two convex lenses to gather and magnify light from distant objects like planets, enhancing distant views effectively.​

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