Re: virtual images and convex lenses

["John S. Denker" <jsd@MONMOUTH.COM>]

Tim Forth wrote:
> I put the screen in the spot where the
> image of the lamp was in focus and then I slid it down to create a split
> screen effect, seeing at the same time both the image projected on the wax
> paper and the magnifying glass directly.
>
> Not only were they the same size, but every edge continued off the paper
> into the lens with nary a break. My eyes were able to focus on both
> simultaneously,

Let me suggest a variation on this experiment:

Use two lamps, one about 2f beyond the glass, and the other
much farther away.  I assume z-2f is more than 1 foot (the
accommodation limit of the eye).

 eye......image1..image2...lens..........lamp1............lamp2

The eye will be able to accomodate one image and then the
other, taking turns, but not both simultaneously.

You will perceive a reasonably natural scene.

You will _not_ however be able to find any particular
place to put the wax-paper screen such that "every edge" will
continue with nary a break.

You will perceive that lamp2 is very, very far behind lamp1,
even though image2 is only a small distance (f) behind image1.

This serves to drive home the point that the perception of
looking through a telescope is very different from the
perception of looking at real images in a camera obscura.