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Re: virtual images and convex lenses



interleaved:

Tim Forth wrote:

> I have two geometrical optics questions.
>
> 1. Why don't you see a real image without a screen? (Actually, sometimes you
> do, why not in this case?)
>
> Imagine this arrangement:
>
> object  ----------------------- focal point ---- convex lens ---- focal
> point -real image ------ eye
>
> That is, a self-illuminated object is placed much greater than the focal
> length (d_object >> f) away from a convex lens. A real image should be
> formed on the other side of the lens. So, if you look towards the lens, why
> don't you see the image without a screen?

you do -- it's out of focus

Try this:  place a piece of tissue paper or lightly ground glass at the "real
image" plane as above (1.); get behind it (away from the object).   see
(observe) the image in focus (If you have presbyopia or normally require
correction use it.)

remove the screen -- no change except better resolution (and luminance).

>
>
> 2. Virtual or real image?
>
> Imagine this arrangement:
>
> object ---------------------------------------------------- f -- convex lens
> -- f --------------eye
>
> When you hold a convex lens far away from your eye and other objects, you
> see an inverted image in the lens. Is this a virtual or real image? What is
> the ray diagram that describes this arrangement?

In the above, the image plane is in the same place as way above.   Try this:
(reverse of the first time)  place your eye as in your 2.  interpose the screen
(translucent)  move it (along the optical axis) 'till you see the image on (in)
the screen.

These demos. are better done in a dark room using a candle, compact
fluorescent, or clear bulb incandescent.

I just did them (demos) using ordinary copy paper, c. fluorescent in chandelier, and one of my 4 D presbyopia lenses to aid in writing this.

I suggest you review geometric optics -- any optics text (I still use Jenkins
and White; H & J is the current one.)

bc          Remember this:  The squeaky person gets the Cleyet.
 

P.s. by definition a real image can appear on a screen, an unreal one can not.

P.p.s.  Reminds me of once when I consulted for (to?) an intraocular mfgr.  Someone made a resolution "tester" using an AF target and a microscope.  They complained that it didn't work very well.  No wonder thought I.  While the company officer looked into the microscope, I whipped away the ground glass screen.  "Whad yuh do?"   "Take a look.", said I.  (adding an achromat collimator also improved it also)

>
>
> Tim Forth
>
> _________________________________________________________________
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