Page 116 - Physics - XII

B′
B
B
F 2F
A
F O Image at infi nity 2F A′ F A O F 2F

(e) Object at F (f) Object between F and O

Fig. 7.1: Formation of images by a convex lens

Image
2F F O
F 2F
Screen
Object
(candle) Convex lens
Fig. 7.2: To study the nature and size of image formed by a convex lens using a candle

Principle/Theory

The size of the image of an object formed by a convex lens depends upon the position of the object w.r.t.
the convex lens. Various images formed for the diff erent positions of the object are shown in Fig. 7.1.

Procedure
1. Estimate the rough focal length of the convex lens.

2. Mount the convex lens in a lens holder in central upright and place it in the middle of the optical bench.
3. Mount the cardboard screen on the right hand side and candle on the left hand side of the lens on the
other two uprights.

4. Arrange the heights of the three uprights in such a way so that the optical centre of the lens, the tip of
the candle fl ame, and the screen lie in a same straight line which is parallel to the optical bench.
5. Displace the candle upright and place it beyond 2F on the optical bench and locate the sharpest real

and inverted image on the screen by adjusting the position of the screen upright. Note the size of the
image, its size is smaller.
6. Now, place the candle upright at the position of 2F and again adjust the position of the screen upright
to get the sharpest, real, and inverted image of same size.

7. Again place the candle upright at the position between F and 2F and adjust the position of the screen
(it is beyond 2F) to get the sharpest, real, and inverted magnifi ed image on the screen.
8. Place the candle upright at F on the optical bench, and adjust the position of screen upright to get

the image.
9. Note all the observations in the observation table.

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