Page 87 - Physics - XII

Procedure

Determinati on of the focal length of the convex lens (f )
1
1. Estimate the rough focal length of the convex lens.
2. Take a plane mirror and clean it. Place it on the horizontal base of an iron stand.

3. Place the convex lens on the plane mirror.
4. Now, fi x the optical needle in the clamp stand and hold it horizontally above the convex lens, so that
its tip lies vertically above the optical centre of the lens as shown in Fig. 7.1. Adjust the height of the
needle equal to rough focal length of the convex lens. By keeping the eye at a distance of about 30 cm
from the needle, look for the image of the needle formed by convex lens and plane mirror.
5. Adjust the position of the needle O so that there is no parallax between the tip of the needle O and its
image I as shown in Fig. 7.1.
6. Using a plumb line and metre scale, measure the distance of the tip of the needle from the centre of the
upper surface of the convex lens. Let it be ‘a’. Now, remove the convex lens and measure the distance
ab
of tip of the needle from the plane mirror. Let it be ‘b’. Then focal length of the convex lens f .
1
7. Repeat the steps 1 to 6 at least two times more and take observations. 2

8. Note all the observations in the observation table.

Determinati on of focal length of the piano-concave lens formed by transparent liquid (water)

9. Fill the open space between plane mirror and lens with transparent liquid (water) using dropper. Now,
place the convex lens over it.
10. Now, repeat steps 4 to 6 to fi nd the focal length of the combination of plano-concave lens (f) and
convex lens (f ). Let a′ and b′ be the distances of the upper surface of the convex lens and the plane
1
mirror from the tip of needle [Fig. 7.2], then the focal length of the combination of convex lens and
plano-concave lens is given by
a'+ b'
F =
2
11. Repeat the steps 9 and 10 at least two times more and take observations.
12. Note all the observations in observation table.
13. After determination of focal length of convex lens f and focal length of combination of convex lens
1
and plano-concave lens formed by transparent liquid F, the focal length of plano-concave lens can

be calculated as
1 1 1
f F f 1

Determinati on of radius of curvature (R) of the surface of convex lens

14. Calculate the pitch and least count of the spherometer.
15. Raise the central screw of the spherometer and place it on the convex surface of the lens so that three
legs of the spherometer rest on it.
16. Now, rotate the screw in downward direction till its tip just touches the convex surface of the lens. In
this position the tip of the screw just touches its faint image on the surface of the lens.

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