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It appears we have not made much progress.
We appear to be talking past each other.
There may be a deep question here, but if so, it is completely
obscured by a bunch of superficial errors. The only way to
proceed is to remove the superficial errors and restate the
question using non-erroneous terminology.
Specifically:
1) The terms "virtual image" and "real image" are very precise
technical terms, which are used incorrectly over and over again
in the message quoted below.
Possibly constructive suggestion: rewrite the passage using
the noncommittal word "scene" instead of "image", except in
cases where the technical term "image" is really truly intended.
2) The human eyeball is not and cannot be a suitable instrument
for "seeing" real images, let alone virtual images. A real image
is formed upon the retina, but only after the light has been
subjected to +25 diopters (or more) of intraocular refraction.
If you want to analyze an optical system, you must analyze the
whole system. The intraocular +25 diopters (or more) is a
verrrry important part of the system in question.
Tim Forth wrote:
I'll recap the situation. I am holding a magnifying glass at arms length
from my eye, looking at a lamp that is 10 feet away. That is, the distance
from eye to magnifying glass is much larger than the focal length of the
magnifying glass, and the distance from lens to the lamp is much, much
larger than the focal length.
That's clear. So far so good.
Let the glass's focal length be denoted by "f".
Let the eye-to-glass distance be denoted by "z".
I wondered why I didn't see the real image of an upside down lamp just
hanging in space. Instead, it looks to me like I am looking through a window
to an upside down world.
Again, the eye is unsuitable for "seeing" real images.
If you want to form a seeable real image, you need to build
a camera obscura, with some sort of screen in the focal plane;
you then look at the image on the screen.
Here's what I think you all were saying: That upside down image in the lens
is in fact a real image.
No. There is no image "in" the lens.
Maybe you are seeing a _scene_ through the lens.
It is in fact hanging in space in front of the
lens, but only looks like it is _in_ the magnifying glass.
There will be a real image hanging in space about one f
in front of the lens, but you have no easly way of knowing
that unless you interpose a screen. It's true that to
form a sharp image on the retina, your eye-lens will need
to refocus ("accommodate") as if it were looking at a point
z-f away, but you will not readily perceive that. Human
depth perception uses many cues, but accomodation is the
_least_ among them.
After first reading this, I found it hard to believe.
What means "this"? What means "it"? I'm missing an antecedent.
It doesn't look like
it is hanging in space, closer to me than the magnifying glass, but you all
seem pretty smart, maybe you all are right. How can I check out this out?
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.
This is an intelligent experiment.
I can follow the description. I assume "screen" means the same
as "wax paper". The result is unsurprising.
If I were right and the image I see in the magnifying glass is different
than the one projected onto a screen, then it ought to be a different size.
I don't understand this. I don't understand the physical
basis for this prediction. I would have predicted just
the opposite.
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, further adding credibility to the idea that the upside down
image that appears to be in the magnifying glass is in fact the real image
made by the magnifying glass.
That conclusion does not follow. Indeed, the conclusion
is not 100% true. That is, imagine somebody standing beside you.
He could see the image of the lamp on the wax paper, even if his
geometry did not permit him to see the lamp through the lens directly.
The wax-paper screen scatters the light. Under the special conditions
such that the screen is in the focal plane, every former outgoing ray
is replaced by an outgoing ray from the same location. Additional
rays (at new, wider angles) are created.
Why, then, would my thumb block the image when I am looking through the
magnifying glass without the screen?
Angles. The wax-paper screen changes the angles.
I think that a carefully drawn ray
diagram shows the answer.
I should think so!!!