Page 157 - NCERT Science Class 10 English Medium

9.3.4 Image Formation by Lenses

Lenses form images by refracting light. How do lenses form images?
What is their nature? Let us study this for a convex lens first.

Activity
Activity 9.129.12
Activity 9.12
Activity 9.12
Activity 9.12
n Take a convex lens. Find its approximate focal length in a way
described in Activity 9.11.
n Draw five parallel straight lines, using chalk, on a long Table such
that the distance between the successive lines is equal to the
focal length of the lens.
n Place the lens on a lens stand. Place it on the central line such
that the optical centre of the lens lies just over the line.
n The two lines on either side of the lens correspond to F and 2F of
the lens respectively. Mark them with appropriate letters such as
2F , F , F and 2F , respectively.
1 1 2 2
n Place a burning candle, far beyond 2F to the left. Obtain a clear
1
sharp image on a screen on the opposite side of the lens.
n Note down the nature, position and relative size of the image.
n Repeat this Activity by placing object just behind 2F , between F
1 1
and 2F at F , between F and O. Note down and tabulate your
1 1 1
observations.

The nature, position and relative size of the image formed by convex
lens for various positions of the object is summarised in Table 9.4.

Table 9.4 Nature, position and relative size of the image formed by a convex lens for various
positions of the object

Position of the Position of Relative size of Nature of
object the image the image the image

At infinity At focus F Highly diminished, Real and inverted
2
point-sized
Beyond 2F Between F and 2F Diminished Real and inverted
1 2 2
At 2F At 2F Same size Real and inverted
1 2
Between F and 2F Beyond 2F Enlarged Real and inverted
1 1 2
At focus F At infinity Infinitely large or Real and inverted
1
highly enlarged
Between focus F On the same side Enlarged Virtual and erect
1
and of the lens as the
optical centre O object

Let us now do an Activity to study the nature, position and relative
size of the image formed by a concave lens.

152 Science

2024-25