Page 117 - Physics - XII

Observations

Rough focal length of the convex lens = F = _____ cm
Position of 2F = _____ cm
Table for the position, nature, size of the image formed by a convex lens with diff erent positions of object

S. Position of
No. Object Image Nature of image Size of image

1. Infi nity F Real and inverted Highly diminished

2. Beyond 2F Between 2F and F Real and inverted Diminished

3. At 2F At 2F Real and inverted Same size

4. Between F and 2F Beyond 2F Real and inverted Magnifi ed

5. At F At infi nity Real and inverted Highly magnifi ed
6. Between F and pole On the same side as object Virtual and erect Enlarged

Result
The change in position, nature, and size of the image is according to the theoretical predictions.

ACTIVITY - 8

Aim

To obtain a lens combination with the specifi ed focal length by using two lenses from the given set of lenses.

Apparatus and Materials Required
A set of thin convex lenses of known powers, optical bench with uprights, screen, a collimator to provide a
parallel beam of light, and a half metre scale

Principle/Theory
Power of Lens
The ability of a lens to converge or diverge the rays passing through it is called the power of the lens (P).
The power of the lens is defi ned as the reciprocal of its focal length.
1
∴ Power, P =
f
The SI unit of power is Dioptre (D).
L 1

F
O O
2 1

f
Fig. 8.1: Parallel beam of light focusing on the combination of two convex lenses

115