Page 118 - Physics - XII

When two or more lenses are placed in contact with each other so that their principal axes is common, then
this arrangement is called the lens combination. If f , f , f , _____ be the focal lengths of individual lenses
3
2
1
and f be the focal length of their combination, then
1 1 1 1 ____
f f f f
1 2 3
So, the power of lens combination is given by

P = P + P + P + _____
1 2 3
where, P , P , P , _____ are the powers of individual lenses.
1 2 3
If two lenses are combined together, then focal length of combination is 1 1 1
f f 1 f 2
The power of this combination is given by, P = P + P
1 2
Procedure
1. First of all, calculate the power of the combination of two lenses corresponding to the required focal
length. Now, select a convex lens from a given set of convex lenses whose power is smaller than that
of the combination of lenses. Combination of
2. Calculate the power of unknown convex lenses L and L Screen
lens to be kept in contact with lens of Collimator Slit 1 2
known focal length to obtain a combination
of lenses of desired focal length. From the
given set of lenses, select the lens whose
power is close to the calculated power.
3. Now, up the optical bench on a horizontal
table. f

4. Place the upright holding a collimator at
one end of the optical bench in such a way
that a parallel beam of light will be directed Fig. 8.2: Experimental setup for fi nding the focal length
along the optical bench. of combination of two convex lenses
5. Place an upright holding the two selected convex lenses L and L on the middle of the optical bench in
1
2
such a way that the two lenses are in contact with each other.
6. Now, place another upright holding a slit in between collimator and lenses in such a way that a parallel
beam of light can easily pass through the slit. Place an upright holding a screen on the other side of the
lenses.
7. Switch ON the collimator so that the parallel beam of light can be directed on the combination of lenses
via a narrow slit.

8. Adjust the distance between the combination of lenses and screen, thereby a sharply focused image can
be obtained on the screen. Measure the distance of screen from the combination of lenses and calculate
their mean.
9. Repeat the above steps at least three times by changing the position of upright holding the lens
combination (say 2–3 cm each time) on the optical bench.

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