Lenses: Definitions, Types and Detailed Explanation

What is a Lens?
A lens can be a plastic or a glass or formed from a combination of these two such that when light passes through it the light rays get refracted.
Light rays from an object present on one side of a lens, when refracted can form an image on the other side of the lens.

Types of lens

There are two known types of lens these are:
  1. A convex/converging lens
  2. A concave/diverging lens


Convex Lens
A convex lens is a type of lens that bends/converges light in, it is also known as converging lens.
The diagram below shows how light is refracted through a convex lens.

Convex Lens



Concave Lens
A concave lens also known as a diverging lens is one that diverges/ bends light outwards.
The diagram below shows the effect on rays being refracted through a concave lens.

Concave Lens



If you look at both convex and concave lens you would notice a difference in their structures. A convex lens has more thickness in its middle than at its edges; this is what aids in converging/ bending light inward. However the concave lens is thicker at its edges than at the middle, which helps to diverge light.


“F” in both diagrams represents focal point. This is the point in a converging lens where the rays converge to and this is the point in a diverging lens such that if the rays where to be traced backward they would meet at this point.
“P” represents the principle axis. This is the imaginary line through the center of the lens.
“C” represents the optical center.
“f” represents the focal length. This is the distance measured from the mirror to the focal point.



Difference between Objects and Images


The object is the body from which the rays of light are emitted before they are refracted. The image is what is formed as a result of the rays being refracted through the lens, they can be seen where the rays of light meet or appear to meet.

Real and Virtual Images

Previously we said that images are formed when rays are refracted through a lens and also that they are formed where the rays of light meet or appear to meet. Because some images refracted actually meet at a point while some really don’t, with this fact we can place images formed into two categories:
  1. Real Images
  2. And Virtual Images
Real images are those that are formed where light rays actually meet after being refracted. These can be captured on a photographic film.
Virtual images, on the other hand, are those that are formed where light rays appears to meet but really don’t after being refracted.


What is Magnification?

The magnification is the ratio of the size of the image to the size of the corresponding object.

The formula below shows the relationship between the magnification, height of the image and height of the object:

Magnification Formula



The formula below shows the relationship between the magnification, distance of image from the lens and distance of object from the lens:

Magnification Formula



Real, Magnified and Inverted


In the example above the image formed is real, magnified and inverted.

Virtual, magnified and erect



Principle of Reversibility

The principle of reversibility states that a ray of light that takes a certain path from A to B will also take the same path from B to A.

The formula below relates focal length, image distance and object distance:

Focal length Formula

The following formula is used to find power of a lens:

Power of a Lens


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