Defence Exam Notes: Formation of Image in a Lens

Images Formed by Lenses

A lens is a part of a transparent thick glass which is bounded by two spherical surfaces. It is an optical device through which the rays of light converge or diverge before transmitting.

Types of Images Formed by Lenses

Lenses are of two kinds: Convex Lenses and Concave Lenses.

Convex Lenses

A convex lens is thicker in the middle and thinner at the edges. A convex lens is also known as a “biconvex lens” because of two spherical surfaces bulging outwards.

Convex lenses include lenses that are plano-convex (i.e. these lenses are flat on one side and bulged outward on the other), and convex meniscus (i.e. these lenses are curved inward on one side and on the outer side it’s curved more strongly).

A concave lens is thicker at the edges and thinner in the middle. A concave lens is also known as a “biconcave lens” because of two spherical surfaces bulging inwards.

Concave lenses include lenses like plano-concave (i.e. these lenses are flat on one side and curved inward on the other), and concave meniscus (i.e. these lenses are curved inward on one side and on the outer side it’s curved less strongly).

Image Formation by Concave Lens

In the case of the concave lens, we always get erect images, diminished images and virtual images.

Object location	Image location	Image nature	Image size
Infinity	At F2	Virtual and Erect	Highly Diminished
Beyond infinity and 0	Between F1 and Optical centre	Virtual and Erect	Diminished

Here if the object is very far away, the images formed by lenses will be all the more diminished.

Image Formation by Convex Lens

In case of a convex lens, if we bring the object close to the lens, the size of the image keeps on increasing. As you bring the object closer to the lens, we get the image all the more enlarged. So here we can say that the images formed can be of a variety of types. We can have diminished inverted image, small sizes inverted image, enlarged inverted image, enlarged erect image. So in a concave lens, there is a possibility of getting a real as well as an inverted image.

Object location	Image location	Image nature	Image size
Infinity	At F2	Real and Inverted	Diminished
Beyond 2F1	Between 2F2 and F2	Real and Inverted	Diminished
Between 2F1 and F1	Beyond 2F2	Real and Inverted	Enlarged
At F1	At infinity	Real and Inverted	Enlarged
At 2 F1	At 2F2	Real and Inverted	Same size
Between F1 and 0	On the same side as the object	Virtual and Erect	Enlarged

Images Formed in Different Lenses

Which lens would you use as a magnifying glass?

Looking at the image, what do you think? What kind of image is formed? Yes, an enlarged image is formed. We cannot get such an image using a concave lens because the concave lens always produces a diminished image. So this is a convex lens.