= Virtual image
{wiki}
A <virtual image> is the apparent origin of an outgoing diverging bundle: the backward extensions of its <light rays> intersect at the image point. Those extensions are geometric constructions, not backward-travelling <light>. A screen placed there cannot intercept the outgoing bundle to display that <virtual image>. An eye or a second <optical lens> receiving the actual outgoing <light rays> can nevertheless form a subsequent <real image>. “Virtual” describes this imaging stage, not whether the image can be seen or photographed. https://tecnoedu.com/recursos/UNLCHidrologia/ManualesOriginalesPasco/Basic-Optics-System-Manual-OS-8515C.pdf[PASCO, Basic Optics System manual, Experiment 14, pp. 41–43: viewing and reimaging virtual images]{ref}.
For a converging <thin lens> with $0<u<f$, the same equations give $v<0$ and $m=f/(f-u)>1$: the <virtual image> is upright and enlarged. For the illustrative choice $f=10\,\mathrm{cm}$, $u=7.5\,\mathrm{cm}$, $h_o=0.30\,\mathrm{cm}$,
$$
v=-30\,\mathrm{cm},\qquad m=4,\qquad h_i=1.20\,\mathrm{cm}.
$$
The image appears farther from the <optical lens> than the object. A diverging <thin lens> with a real object instead has $v<0$ and $0<m<1$, so a <virtual image> need not be enlarged. A plane mirror provides another <virtual image>, located behind the reflecting surface by backward extensions of the reflected <light rays>. https://openstax.org/books/college-physics-2e/pages/25-7-image-formation-by-mirrors[Urone and Hinrichs, College Physics 2e, §25.7: Plane-mirror images]{ref}.
\Image[/image-magnification-virtual-rays.png]
{title=Backward ray extensions locate a <virtual image>; $u=f$ gives parallel outgoing <light rays>}
{description=Original ideal-model diagrams for a converging <thin lens> with $f=10\,\mathrm{cm}$ and $h_o=0.30\,\mathrm{cm}$. Above, solid <light rays> travel rightward while dashed backward extensions meet at $(-30,1.20)\,\mathrm{cm}$. Below, each object point at $u=f$ sends out a parallel bundle, so there is no finite image position. Vertical distances are enlarged; both panels share coordinate scales. These are predictions, not measurements.}
For a direct viewing test, bring the target to $u=7.5\,\mathrm{cm}$, search for a sharp screen image on the outgoing side, then remove the screen and look through the <optical lens> toward the target. Expect an upright enlarged view if the eye receives the bundle and can focus. Record the eye position: $5\,\mathrm{cm}$ beyond the <optical lens> gives a predicted apparent distance $35\,\mathrm{cm}$. This adapts the Northern Virginia laboratory's comparison. https://sites.nvcc.edu/alphy/_files/Manuals/Physics%20242%20Lab%2011%20-%20Thin%20Lenses.pdf[Northern Virginia Community College, Physics 242 Lab 11, pp. 4–5: Virtual images with a converging lens]{ref}.
For a position measurement, use <parallax localization of a virtual image>. Place a movable pencil tip or narrow comparison marker beside the apparent image, and view that marker directly beside or above the <optical lens>, while viewing the image through it. Move the head a little sideways and move the marker along the bench until relative lateral motion vanishes. Read its coordinate $x_m$; the estimate is $v=x_m<0$. Deliberately place the marker nearer and farther to check that relative motion reverses on opposite sides of the match. This follows Orhan Aytür's lens laboratory, which uses a pencil to locate a <virtual image>. https://www.ee.bilkent.edu.tr/~ee428/manual/html/node6.html[Orhan Aytür, Bilkent University, Experiment 2: Imaging with mirrors and lenses, §2]{ref}.
Keep target and <optical lens> fixed, use one eye and the central image detail, and ensure the marker itself is not viewed through the <optical lens>. A marker blocked by the target, or a head movement that loses the bundle through the <aperture>, invalidates that trial. Repeat the match from both directions and record its depth range. This measures apparent depth; comparing apparent sizes at different depths does not directly measure <transverse magnification>. Near $u=f$, apparent depth becomes very large and the match is poorly constrained. A failed screen search alone therefore needs the distance check and the viewing test.
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