Question

the expression for astronomical telescope​

Answer 1

Answer:

The distance b/w objective lens and the eye piece is equal to the length of the telescope. From figure: OE = length of telescope =L

But OE = OB + BE OB = Fo & BE = Fe

OR

OR

Explanation:

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Answer 2

Answer:

Where, O→ Objective lens, O′→ Eye lens, PQ→ Image

(ii) Expression for magnifying power: If angle made by an object at least distance of distinct vision is α and angle made by an image is β. Then magnification.

m=αβ               .........(i)

Object is situated at infinity, therefore angle subtended by an object on objective lens is said to be angle subtended by an object on eye.

Now, tanβ=O′Q′P′Q′ and tanα=OQ′P′Q′

As α and β are very small hence,

tanβ≈β and tanα≈α

∴β=O′Q′P′Q′ and α=OQ′P′Q′

Putting the values of β and α in equation (i), we get

m=P′Q′/OQ′P′Q′/O′Q′=O′Q′OQ