A tower 100m tall at a distance of 3km is seen through a telescope having objective of focal length 140cm and eyepiece of focal length 5cm. What is the size of final image if it is 25cm from the eye?

A tower 100m tall at a distance of 3km is seen through a telescope hav

1.	A tower 100m tall at a distance of 3km is seen through a telescope having objective of focal length 140cm and eyepiece of focal length 5cm. What is the size of final image if it is 25cm from the eye?
Answer» SOLUTION :For objective LENS `1/v-1/(3 times 10^5)=1/140` i.e., `v=140 cm =f_0` So `m_0=v/u= 140/(-3 times 10^5)=- 14/3 times 10^-4` and as FINAL IMAGE is at least distance of distinct vision so for eye lens, we have `1/(-25)-1/u_e=1/5 ` i..e, `u_e=- 25/6` So `m_e=v_e/u_e= (-25)/((-25/6))=6` and hence, `m=m_0 times m_e= - 14/3 times 10^-4 times 6` But as `m=(I/O)` `I=m times O= -28 times 10^-4 times (100 times 10^2)=-28 cm` Negative sign implies that image is inverted.