Page 20 - 10-phy-12 Geometrical Optics

remove the parallax between the image and image needle, by moving
this image needle back and forth.

8. Note the positions of the object needle upright, image needle upright
and lens upright.

9. Calculate the distance between the object ! CAUTIONS
needle and lens i.e., p= OL and the distance

between the image needle and the lens i.e., Ÿ The lens should be

q=IL. cleaned with a cotton
10. Calculate the focal length of the given lens cloth or with a tissue
paper.

using the formula Ÿ All the upright

1= 1+ 1 needles should be
f pq vertical.
Ÿ Parallax should be
removed carefully for

11. Take three readings by increasing the object each reading.

distance in regular steps without changing

the position of the lens.

Approximate Focal Length. = fa= ………………………………cm.

No. of Obs. Position of Distance Focal length

Observations Object Lens Image pq f = pq
and Data needle L needle p+q
Processing (cm) (cm)
O (cm) I (cm)

(cm) (cm)

1

2

3

Mean focal length = …………………… cm

Conclusions

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