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Lens Theory

Lenses are essentially curved-side magnifying glasses. With appropriate sign conventions, the lens formula can be used in any context. This lens formula works for both concave and convex lenses. The image is a virtual picture on the same side of the lens as the object if the equation yields a negative image distance. If the focal length in this equation is negative, the lens is a diverging lens rather than a converging lens.

Lens Formula

Lens formula is given as:     

1/a-1/b=1/f 

Where,

a = distance of the image from the lens

b = distance of the object from the lens

f = focal length of the lens

This lens formula is applicable for both concave and convex lenses.

Introduction

A lens is a piece of transparent glass that concentrates or disperses light rays as they pass through it. Lenses are employed in telescopes and other magnifying devices because of their magnifying properties. The Lens formula describes the relationship between the distance of an image (a), the distance of an object (b), and the focal length (f) of the lens in optics.

Types of lens

A lens is just a piece of transparent material that has been finely shaped to refract light rays into an image. Lenses are a collection of small refracting prisms, each of which refracts light to create its image.

There are different types of lenses:

Non-spherical lenses

  1. Aspheric lens: A lens with an aspheric surface is one whose surface is not shaped like a sphere or cylinder. It is also known as a non-spherical lens.
  2. Cylindrical lens: Lenses that have a curvature along only one axis are classified as cylindrical lenses. Their main purpose is to convert laser diode elliptical light into a round beam or to focus light into a line. 
  3. Fresnel lens: The optical surface of a Fresnel lens is divided into narrow rings. As a result, the lens can be significantly smaller and lighter than traditional lenses.
  4. Lenticular lens: Lenticular lenses are microlens arrays used in lenticular printing to create pictures that appear to have depth or alter when viewed from various angles.
  5. Bifocal lens: Two or more focal lengths, or a graded range, are built into a bifocal lens.
  6. Gradient index lens: The optical surfaces of a gradient index lens are flat, but the index of refraction varies radially or axially, causing light traveling through the lens to be focused.

Convex lens

The convex lens converges light rays that travel parallel to its primary axis and is comparatively thick in the middle and thin at the lower and upper edges. Instead of curving inside, the edges curve outward.

When parallel light beams pass through a convex lens, the refracted rays converge at a single point, defined as the principal focus.

The focal length is the distance between the principal focus and the lens’s center.

Use of convex lens

Convex lenses are used for various purposes in our daily life. They are as follows:

  1. The most common use of convex lenses is in human eyes which helps us to see.
  2. Convex lenses are also used as magnifying glasses.
  3. Hypermetropia, or long-sightedness, is treated with the help of convex lenses.
  4. Because it focuses light and generates a clear and crisp image, it is utilized in cameras.
  5. Convex lenses are also used in magnifying devices such as microscopes, telescopes, and camera lenses.

Concave lens

The middle of concave lenses is thinner. The light rays that travel through the lens are dispersed i.e. they are diverging in nature. A diverging lens is a concave lens.

A concave lens has at least one inwardly curving surface. It is a divergent lens, which means that light rays refracted via it are stretched out.

When parallel light beams pass through a concave lens, the refracted rays diverge and appear to originate from a single point known as the principal focus.

The focal length is the distance between the primary focus and the lens’s center.

Use of concave lens

  1. Concave lenses are used as spy holes in the doors.
  2. Myopia, often known as nearsightedness, is treated with concave lenses.
  3. Concave lenses are also used in telescopes and binoculars for focusing the image and making it more clear.
  4. To focus on the specific object, concave lenses are used in cameras for a clearer view.
  5. Concave lenses are also used in flashlights.

Conclusion 

As we discussed above, a piece of transparent material (such as glass) with two opposite regular surfaces, one curved and the other plane, is used separately or in combination in an optical device to generate an image by concentrating light rays. The above-given lens formula is applicable for both convex and concave lenses. Both lenses behave differently, one is converging in nature and another one is diverging in nature. Both lenses are used in our daily life for different purposes.