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To bbtiin a lens combination with the spe fied focal length by using two lenses
DE From the given set of lenses.
APPARATL 1S AND MATERIAL
Is Apparatus. No particular apparatus is needed
< lenses, one of these is of given focal length (sa
uch that the combination gives a single lens of focal length
ertical wooden board with broad stand,
Material. A set of thin conv
(we have to select a second let
0 cm), lens holder with stand, a white painted v
f
half r
PHEORY
, 1. The reciprocal of focal length in metre is called power of lens in dioptre (D).
1
pai
f
2 With a convex lens, the real image of a distant object is formed at a distance equal to
its focal length.
3. If, and f, be the focal lengths of the two lenses and F be the focal length of the
combinatio
1o1it
.
Then, aos
Fh kh
nd combination of power P.
For lenses of power P, and P,
Then, P=P,+Py
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221
Ack
L
ROCEDURE
Measurement of focal length
c sep the white pai
1. Keep the white painted vertical wooden board to serve as a screen.
2. 'The convex lens vn foc 5
mw oe lens (known focal length f, = 15 em), fixed into a holder stand is put on the
eft of the screen. There are sunlight illuminated green trees at large distance on the
left of the lens.
3, The lens is moved towards and away from the sereen tilla
sharp, inverted image of
tree
4. Distance between ce}
metre scale.
5. ‘The distance gives the foca
6, Replace first lens by se
2 to 5. This gives the focal
h lenses in contact al
is formed on the screen,
tral lines of the sereen and holder stand is measured by a half
| length of the convex Jens about 15 cm.
; of required power and repeat the steps from
cond convex lens
| Jength of second convex lens.
7, Now bring both nd repeat the steps from 2 to 5. This gives the
combined focal length.
8. Determine the
the focal length
focal length with other given lens.
» of about six of the
; convex lenses
Determine
"AL.CULATIONS
100
F(cm)
ZL tet P= 106m, so that P= 10D (- Pw)
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sy
COMPREHENSIVE PRACTICAL Pusicg_,
; BONE PRACTICAL PHYSIC)
Following combinations will be suitable. _
Power Focal length
Py) PyD) filem) frlem)
| 2 a 50 | 12.5 |
|
5 | 5 20 2 |
4} _____1_______}
(Note. The ideal values are as sample.)
VERIFICATION
k
PRECAUTIONS
& 1. Thin lenses should be taken.
2. Lenses should have same aperture.
SOURCES OF ERROR
K 1, Lenses may not be thin,
2. Lens apertures may not be same.
The above combinations may be tried and result verified.
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