Page 119 - Physics - XII

Observations

Least count of scale of optical bench = _____ mm
Focal length of fi rst lens L = f = _____ cm
1 1
Focal length of second lens L = f = _____ cm
2
2
ff
Focal length of lens combination, f 12
f f
1 2

Table for Focal Length of Combination of Two Convex Lenses

S. Distance of lens L from Distance of lens L from Mean distance,
2
1
No. the screen, d (cm) the screen, d (cm) d = d + d 2 = f (cm)
1
2
1
2
1. f = _____ cm
1
2. f = _____ cm
2
3. f = _____ cm
3

Calculations

Find the focal length for each set of observation then take the mean of all values of focal length.
f f f
Focal length, f 1 2 3 _____ cm
3
Result
1. Focal length of combination of two lenses having focal lengths of f and f in contact,
1
2
ff
f 12 _____ cm
f f
1 2
2. Measured value of focal length of lens combination = _____ cm

3. The diff erence between measured value and calculated value of focal length (or error in the measured

value of focal length) = _____ cm

Precautions
1. Both the lenses should be thin and they should have the same aperture. Also, the lenses should be
placed in such a way that their axes lie on the same straight line. Before using the optical bench, it

should be set horizontally by using spirit level and levelling screws.
2. For each set of observations, the change in position of upright holding the lens should be very small
(say 2 to 3 cm only).
3. The power of the selected convex lens should be smaller than that of the combination of lenses.

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