# Image Formation by Lenses

Lenses form images by refracting light.

Image formation by convex lens:

Take a convex lens. Find its approximate focal length.

Draw five parallel straight lines on a table such that the distance between the successive lines is equal to the focal length of the lens.

Place the lens on the central line such that optical centre of the lens lies just over the line.

The two lines on either side of the lens correspond to F and 2F of the lens respectively. Mark them 2F1, F1, F2 & 2F2 respectively.

Place a burning candle far beyond 2F1 to the left. A clear sharp image is formed on a screen placed at F2.

Repeat this Activity by placing object just behind 2F1, between F1 & 2F1, at F1, between F1 & O.

Nature, position and relative size of the image formed by a convex lens for various positions of the object

 Position of the object Position of the image Relative size of the image Nature of the image At infinity At F2 Highly diminished, point-sized Real and inverted Beyond 2F1 b/w F2 & 2F2 Diminished ” At 2F1 At 2F2 Same size ” Between F1 & 2F1 Beyond 2F2 Enlarged ” At focus F1 At infinity Infinitely large or highly enlarged ” Between F1 & O On the same side of the lens as the object Enlarged Virtual and erect

Image formation by concave lens:

Place a burning candle on one side of a concave lens.

Look through the lens from the other side and observe the image. Image will not be obtained on a screen but can be observed through the lens.

Move the candle far away from the lens. The image becomes highly diminished.

Nature, position and relative size of the image formed by a concave lens for various positions of the object

 Position of the object Position of the image Relative size of the image Nature of the image At infinity At F1 Highly diminished, point-sized Virtual and erect b/w infinity & O b/w F1 & O Diminished Virtual and erect

A concave lens always gives virtual, erect & diminished image, irrespective of the position of the object.

# Image Formation in Lenses Using Ray Diagrams

For drawing ray diagrams in lenses, any two of the following rays are considered:

a)   A ray of light parallel to the principal axis: After refraction from a convex lens, passes through the principal focus on the other side of the lens. For a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens.

b)   A ray passing through a principal focus: After refraction from a convex lens, emerges parallel to the principal axis. A ray of light appearing to meet at the principal focus of a concave lens, after refraction, emerges parallel to the principal axis.

c)    A ray passing through the optical centre: It emerges without any deviation.

The ray diagrams for position, size and the nature of the image formation in a convex lens for a few positions of the object are shown below (Refer the table):

 (a)

 (b)

 (c)

 (d)

 (e)

 (f)

The ray diagrams for the image formation in a concave lens for various positions of the object (Refer the table):

# Sign Convention for Spherical Lenses

It is same as the sign convention used in spherical mirrors.

The rules are applied for signs of distances, but the measurements are taken from optical centre of the lens.

According to the convention, focal length of a convex lens is positive and that of a concave lens is negative.