# Geometric Optics

is it possible to have an virtual OBJECT in image formation?

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Danger
Gold Member
I don't quite understand the question. Unless there's technical terminology that I'm unaware of, an image is a virtual object.

I think yes, with two lenses (esp. one diverging), I think you might use one lens to create an image, then put the second lens *before* the image, so that the image for the first lens does not actually form, but acts as a virtual object to the second lens.

I think you might use one lens to create an image, then put the second lens *before* the image, so that the image for the first lens does not actually form, but acts as a virtual object to the second lens.
I also thing this is what the OP is refering to, but instead of describing this as a real process I would say it is a valid method of geometric optics (it is not the only method).

Unless there's technical terminology that I'm unaware of, an image is a virtual object.
Alas, there is technical terminology in basic (ie. ray) optics (in this case relating to the sign of variables in the lens maker equation) particularly the terms image vs object, real vs virtual, erect vs inverted. An image is only real if you see it when you put a screen there, if you can't do that (typical example is how light seems to emanate from an apparent image *behind* a mirror, where there is no light)

I also thing this is what the OP is refering to, but instead of describing this as a real process I would say it is a valid method of geometric optics (it is not the only method).
What do you mean Crosson? In particular, what do you mean by "real process", and what other method are you thinking of?

What do you mean Crosson? In particular, what do you mean by "real process", and what other method are you thinking of?
I think of geometric optics as a method for solving for the image of various lens configurations. A final image of an object through configuration of two lenses can be analysed in two ways:

1) Follow key rays through lens1 and use geometry for the (difficult) analysis of these rays when they pass through lens2 (they will not, of course be key rays of lens2).

2) Find the image of the object through lens1, and use this image as the object for lens2 to generate the final image.

There is no question that 1 is valid, but 2 seems more questionable but indeed seems to be valid (although I have never seen a proof of this).

Then I made a comment about the realist interpretation of 2, i.e. did it really happen or is 2 just a method. I don't know.

2 is the correct way.

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Danger
Gold Member
Thanks for the explanation, CF. I think that I'll just stay as far away from optics as I can; it's way too complex. (I have a book about it, but it doesn't help much. )

I think the problem is just that there are lots of poor ray-diagrams around. If you learn to draw them well, then ray optics is fairly trivial (for example, the lens maker equation is just trigonometry), but if you don't have those tricks/concepts explained/corrected, most students really struggle.

This happens to be the one topic where I've experimented with teaching methods: this may be anecdotal, but after getting every student in a tutorial to contribute in turn to part of a series of ray diagrams on the whiteboard, they performed significantly above average on that topic. Unfortunately I don't know how to apply that technique for teaching other topics, where in practice the problems require a deeper plan.

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Danger
Gold Member
I never realized that you're a teacher; for some reason I assumed that you were a kid (albeit a very well-educated kid).
I can't even begin to understand that book. It's from the 60's or maybe 70's. There are an awful lot of drawings, for sure, but essentially it's like one long equation from beginning to end. There are symbols in there that I don't even know the meanings of, like integrals and such. I remind you that I have a grade 9 math education. I can do simple geometry, but don't know any trig.

I never realized that you're a teacher; for some reason I assumed that you were a kid
For the record I'm a just bit of both: PhD student, with some first year tutorials etc on the side.

Danger
Gold Member
Impressive enough for me.