On Reversing The Telescope The Image Is Too Small Why?

[akingdangi2]

It is a question of physics 12 class

[ToraToraTora]

it's the opposite of why things are too big when it's the correct way round.

[Old_Geezer]

It's what lenses do, they bend light.

[Huderon]

I hope it wasn't urgent homework, but I'd have expected at least one of the physics whizzes to come up with a sensible answer.

Basically it's down to the focal lengths of the two lenses. If the front lens (the objective) has a focal length of, say, 500mm and the eyepiece has a focal length of 10mm, you work out the magnification by dividing the focal length of the objective by the focal length of the eyepiece - in this case 500/10 = 50. the thing you are looking at is magnified 50 times.

Now turn the telescope around so that the eyepiece is now the objective and the objective is now the eyepiece and do the same division 10/500 = 0.02 or to put it another way, blimey that's a seriously small image !

[Peter Pedant]

hows this for a whizzy answer

draw a standard ray diagram of a star being magnified
O --> I ( no great magic there) O is small ( star) and I is large (image)
then show that ray tracing in optics is reversible
which at your stage you can just say - looking at the diagram it is obvious that it does
so I is now the object and O is now the image and that is much smaller

( and you dont even have to get your knickers in a knot over real-is-positive or NCC ( new cartesian coordinate convention)

[Huderon]

Ah ! Ray tracing !

Wot dat den ? (sorry PP, I just could not resist the temptation)

I'm guessing that it is the explanation for why the focal lengths bit works, but it's mumbltey years since physics classes and I haven't been able to find my catechism of natural philosophy for years, so I guess I'd better look for an idiots guide and see if I can figure it out.