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A common method to solve problems in optics for system of mirrors, surfaces with different refractive index, lens, slabs, mixture of mirror and lens etc. is to consider the image formed by the first optical device as the object for the other and so on till we reach the final image. Questions like I posted below. enter image description here

Now my question is, what happens to these intermediate images like I1 in the above picture. Won't many images be formed if we have system of many lens/mirrors? If yes then to which observer are these images visible to? Which observer is able to see the final image and which observer will see all the images?

I asked my teacher for the same and he just told me that, "WE ARE NOT THE OBSERVERS". So If we aren't the observers then who is?

Also what is the criteria of seeing? Why can't we see the object (pencil here) if it is physically placed in the water like in the picture I posted below. Why only we see the refracted image? enter image description here

Please help, thanks.

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2 Answers 2

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It depends on where the observer's eye is

enter image description here

The observer's eye is usually at $B$, but if it were at $I_1$ with the $f_2$ lens removed, or at $C$, then they would see the light that forms, or would go to form, the other image.

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  • $\begingroup$ Thankyou so much. $\endgroup$ May 16 at 11:24
  • $\begingroup$ The chances are that putting the eye at position B will not enable one to see a sharp image of the object. $\endgroup$
    – Farcher
    May 16 at 13:55
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. . . what happens to these intermediate images like I1 in the above picture?

If lens $L_2$ was not there a real image would be formed at $I_1$ but with lens $L_2$ in position that image would not be formed rather the light would be focussed by lens $L_2$ to form an image at $I_2$.
As drawn there is no way of putting a screen at position $I_1$ and seeing an image which is formed by light only passing through lens $L_1$.

Why can't we see the object (pencil here) if it is physically placed in the water like in the picture I posted below?

The diagram is misleading in that it is not showing what a observer would see.
Just try it yourself!

enter image description here

Note that the "bending" effect is not seen sideways on (left image) but is seen from "above" (right image).

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