if the focal length of objective lens of an astronomical telescope is 20 cm and the length of it is 25 CM the magnification of the telescope for normal adjustment is
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Answer:L=100cm
f
0
+f
e
=100 .....(1)
m=
f
e
f
0
19=
f
e
f
0
⇒ f
0
=19f
e
Substituting in Equation (1)
19f
e
+f
e
=100
f
e
=5cm
∴f
0
=19×5
So, f
o
=95cm
Explanation:
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Using the lens law formula.
- 1/f=1/v-1/u
- Given f=20, u=25
- Put in above formula
- 1/20=1/v-1/25
- 1/20+1/25=1/v
- Taking LCM
- (5+4)/100=1/v
- v=100/9
- Now the magnification M=v/u
- M=(100/9)/25
- M=4/9
- M=0.44 is the magnification of the telescope.
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