Page 23 - 10-phy-12 Geometrical Optics

Materials Optical bench, a lens of focal length10 cm (eye-piece), a lens of focal length 5 cm
Required (objective), two lens holder uprights a meter rod and two upright needles

Procedure 1. Find approximate focal lengths of both the lenses. Record these focal
lengths as fo and fe (fo< fe).

2. Install the lenses in their uprights (universal lens
holders).

3. Place a needle upright near one end of the

optical bench. This is the object OO′. Why the objective lens of

4. Place the lens with shorter focal length microscope has short
(objective lens) in front of the object needle at a focal length and not a
distance slightly greater than its focal length. long one?

5. Use second needle to locate position of a real
image II′ of the object OO′by removing parallax.

6. Measure distance from the objective lens to the image needle.

7. Now remove the image needle from the optical bench.

8. Place the second lens of larger focal length (eyepiece lens) on the
optical bench just beyond the image position

9. Look through this eyepiece and adjust it to observe an inverted but
magnified final image II² of the needle.

Objective I Eyepiece Eye
O Lens
Lens Magnified image II’’
O’
I¢

0 10 20 30 40 50 60 70 80 90 100

Fig. 12.6a.3

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