Lens: Types, Formula, Image Formation & Uses | Physics

A lens is an optical transmissive device that converges or diverges beams of light through refraction. A simple lens is one piece of transparent material. On the other hand, compound lenses are made up of a number of simple lenses placed along a common axis.

Important Terms

Principal Axis: The straight line that goes through the middle of the lens and the two focal points.
Optical Centre (O): Middle point of the lens through which light travels undeviated.
Focus (F): Where rays either converge (convex) or seem to diverge from (concave).
Focal Length (f): Distance between the optical centre and the focus.
Centre of Curvature (C): The centre of the sphere of which the lens surface is a portion.
Aperture: The effective lens diameter through which light enters.

Types of Lenses

Convex Lens (Converging Lens)

It is thicker in the middle and thinner at the edges. It converges (brings together) parallel rays of light to a point referred to as the focus.

Concave Lens (Diverging Lens)

A concave lens is thinner in the middle and thicker at the edges. It diverges (spreads out) parallel rays of light, seeming to come from a point referred to as the focus.

Image Formation by Convex Lens

Position of Object	Position of Image	Nature of Image
At infinity	At focus (F)	Real, inverted, highly diminished
Beyond 2F	Between F and 2F	Real, inverted, diminished
At 2F	At 2F	Real, inverted, same size
Between F and 2F	Beyond 2F	Real, inverted, magnified
At F	At infinity	No image (parallel rays)
Between O and F	On the same side of the lens	Virtual, erect, magnified

Image Formation by Concave Lens

A concave lens always forms a virtual, erect, and diminished image on the same side of the object.

Lens Formula

The relation between object distance (u), image distance (v), and focal length (f) is given by:

Where:

f – focal length of the lens
v – image distance from the optical centre
u – object distance from the optical centre

Magnification by a Lens

Magnification (M) is given by:

If M > 1 → Image is magnified
If M < 1 → Image is diminished
If M = 1 → Image is of the same size

Positive magnification → Virtual and erect image
Negative magnification → Real and inverted image

Power of a Lens

The power (P) of a lens is its capacity to converge or diverge light and is expressed in diopters (D).

A convex lens has positive power.
A concave lens has negative power.

Uses of a Lens

Microscopes – Convex lenses enlarge small objects.
Cameras – Lenses focus light to form images.
Binoculars – Used to view distant objects.
Projectors – Project images onto a screen.
Magnifying glasses – Enlarge nearby objects.
Flashlights – Focus and direct light beams.
Peepholes – Provide a wide field of view.

Summing Up

A lens is a transparent optical device that bends light to form images. Convex lenses converge light and can form real or virtual images, while concave lenses diverge light and always form virtual, erect, and diminished images. Lens behaviour is governed by the lens formula, magnification, and power.