draw a labelled ray diagram to show the formation of image in an astronomical telescope in the normal adjustment position and write the expression for its magnifying power.

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The telescope is said to be in normal adjustment when the final image is formed at infinity.



The magnifying power under situation is as follows.


Magnifying power is the ratio of the angle subtended at the eye by the final image(β) to the angle subtended at the eye by the object directly(α), when both the final image and the object lie at infinite distance from the eye.


m = β/α = tanβ / tanα

tanβ= AB / AO2

tanα = AB / AO1

Therefore m = AB x AO1 / AB x AO2 = AO1 / AO2 = fo / -fe

m = -fo / fe

Negative sign shown that final image is inverted.

Thus in normal adjustment magnifying power is the ratio of focal length of objective to the focal length of eyepiece.

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