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Surveying - Engineering

Q1:

In a telescope the object glass of focal length 14 cm, is located at 20 cm from the diaphragm. The focussing lens is midway between them when a staff 16.50 m away is focussed. The focal length of the focussing lens, is

A 5.24 cm

B 6.24 cm

C 7.24 cm

D 8.24 cm

ANS:C - 7.24 cm

Let me explain, also tell me if I'm wrong?

1/F1=((1/14)+(1/20))=1/.1214 = 8.237.
1/F2=,((1/16.5)+(1/10))=1/1606 = 6.226.
Note- 10 is taken because its in midway of 20.
Now take the average for F=(f1+f2)/2.
= 7.24. I/f = 1/ f1 + 1/f2.
= 1/14 +1/16.50.

1/f = 0.314.
F = 7.24.