Wednesday, September 28, 2022

10. Light - Reflection and Refraction | Class 10/ CBSE | Web Notes | Part 8: Image Formation by Lenses

10. LIGHT – REFLECTION AND REFRACTION

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.


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