Lenses

A transparent medium bounded by two surfaces of which at least one is spherical is called a

It can classified into two types

Convex LensesConcave Lenses

Convex Lenses

Thicker in the center than edges. – Lens that converges

(brings together) light rays.

– Forms real images and virtual images depending on position of the object

The Magnifier

Concave Lenses

• Lenses that are thicker at the edges and thinner in the center. – Diverges light rays – All images are

erect and reduced.The De-Magnifier

Term Related to lenses

Principal axisOptical centrePrincipal focusFocal length

The line joining the centres of the two spheres C1C2 of which the lenses form a part is called the principal axis.

The point o is called the optical centre of the lens thus, the optical centre of a lens is a point on the principal axis suchThat a ray passing through it does not undergo any deviation.

The point at which all rays parallel to the principal axis of a spherical mirror of a lens meet after reflection or bendingInwards.

The distance between the optical centre and the focus of a lens is called the focal length of a lens.

A ray parallel to the principal axis :In the case of a converging lens, after refraction, this ray will pass through the focus. In case of a diverging lens, after refraction, it will appear to come from the

focus.

A ray passing through the optical centre

This will emerge undeviated

A ray passing through the focusThis will emerge parallel to the principal axis.

The image formed by a convex lens varies in size, Position and nature (erect or inverted, real or virtual ) Depending upon the distance of the object from the lens.

Name of the object

Position of the image

Nature of the Image

Size of the Image

Very Far off

Beyond 2F

at 2FBetween F and 2Fat F

Between F and 0

at FBetween F and 2F

at 2F

Beyond 2F

at infinity On the same side as the object

Real and invertedReal and invertedReal and inverted

Real and inverted

Real and inverted

Virtual and erect

Smaller than the object

Smaller than the object

Same size as the object

Larger than the objectMuch larger than the object

Larger than the object

The Rays falling on a concave lens, after refraction, always diverge. Therefore, no matter where the Object is kept, a diverging lens always form a Virtual image, which is erect and smaller in size Than the object . It is formed between the opticalCentre and the focus, on the same side of the lensAs the object.