Virtual Vs Real Image: How Mirrors and Lenses Form Each Type, and How to Tell Them Apart

Short answer

• A real image forms where light rays actually meet after reflection or refraction. Because real light passes through that point, a real image can be projected onto a screen placed there. A virtual image forms where light rays only appear to meet. The rays leaving the mirror or lens are spreading apart, and if you trace them backward with dashed lines, those extensions meet at a point where no light actually travels. You can see a virtual image by looking into the mirror or through the lens, but you cannot project it onto a screen, because there is no light at the virtual image point to catch. That single test, whether the rays really converge or only appear to converge when traced backward, is the whole distinction. Size, brightness, and whether the image looks convincing are not the test. A virtual image in a good mirror can look perfectly solid, and a real image on a screen can look faint. What matters is what the light is doing. In the single mirror and single lens cases in this article, real images are inverted, and virtual images are upright. That pattern is a useful check, but it is not the definition. The definition is convergence versus apparent convergence.

The One Test That Decides It

Imagine one point on an object, for example the tip of an arrow. Light leaves that point in many directions. A mirror or lens redirects some of those rays. After the mirror or lens, only two things can happen to rays that started from the same object point.

In the first case, the redirected rays bend toward each other and cross at a single point. That crossing point is the image of the object point. Do the same for every point on the object, and you get the full image. This is a real image. A screen, a piece of film, or a camera sensor placed at the crossing point will receive light there, so the image appears on it.

In the second case, the redirected rays spread apart after the mirror or lens. They never cross. If you extend them backward, in the direction opposite to their travel, the extensions cross at a point behind the mirror or on the same side of the lens as the object. Your eye follows the rays backward in exactly that way, so your brain places the image at that apparent crossing point. This is a virtual image. No light reaches that point, so a screen placed there stays blank.

Ray diagrams make this distinction visible. Real rays are drawn as solid lines. Backward extensions are drawn as dashed lines. A point where solid lines cross is a real image point. A point where only dashed lines cross is a virtual image point. Learning to read that dashed versus solid difference is the fastest way to read any ray diagram with confidence.

What a Real Image Is

A real image is an image formed by actual convergence. Light from an object point is redirected and then passes through the image point. The image therefore has a real, measurable location in space, on the side where the light actually travels after the optical element.

For a lens, the real side is the side opposite the object. Light passes through the lens and converges beyond it. For a mirror, the real side is the front of the mirror, the same side as the object, because reflected light stays in front of the mirror. This difference in side trips up many students, so it is worth stating plainly: opposite side for a lens, same side for a mirror. The test is still convergence, not side alone.

Real images can be larger than the object, smaller than the object, or the same size, depending on the device and on where the object sits. In the single element cases covered here, real images are inverted, meaning upside down relative to the object. A projector gives the everyday example. A projector lens forms a real, inverted, magnified image on a screen, and the projector is arranged so the final picture on the screen reads the right way up. A camera works the same way in principle. Its lens system forms a real image on film or on a sensor, where it can be recorded. Your eye also forms a real image. The lens system of the eye brings light to a focus on the retina, and the retina acts, in effect, as the screen.

A real image does not need a screen to exist. The rays still cross at the image point whether or not a screen is there. The screen only makes the crossing visible by scattering the light at that plane. If you put your eye at the right place beyond a real image, you can also see it directly, because your eye then forms its own real image of that light on your retina. The screen test is a test of convergence, not a requirement for the image to exist.

What a Virtual Image Is

A virtual image is an image formed by apparent divergence. Light from an object point is redirected so that the outgoing rays spread apart, and those rays appear to come from a point that the light never visits. Your eye receives the spreading rays, and because your visual system assumes light travels in straight lines, it places the source of those rays at the point where their backward extensions meet.

The most familiar virtual image is your reflection in a flat bathroom mirror. No light passes behind the mirror, and nothing is actually standing behind the glass. The reflected rays reach your eye from the mirror surface, and they behave exactly as if they had come from a person standing behind the mirror. That is why the image looks located behind the mirror, at the same distance behind it as you stand in front of it.

Virtual images are not fakes, illusions in the sense of tricks, or images that do not count. They are the normal result of how eyes interpret diverging light. A magnifying glass gives another common virtual image. Hold a converging lens close to small print, within its focal length, and the print appears larger and upright, on the same side of the lens as the print itself. The lens has not created a new object. It has changed the direction of the rays so that your eye sees a magnified, upright, virtual image.

In the single mirror and single lens cases covered here, virtual images are upright, meaning the same way up as the object. They may be the same size as the object, larger, or smaller, depending on the device. A flat mirror gives a same size virtual image. A magnifier gives an enlarged virtual image. A convex mirror gives a diminished virtual image. Orientation alone can therefore suggest the answer, but the convergence test remains the definition.

Real Image Compared With Virtual Image

The table below summarizes the distinction for a single mirror or a single lens acting on a real object. Combined systems, with two or more elements, follow the same rule element by element, but the image of one element can become the object for the next, so the simple table should not be applied to the whole system at once.

| Feature | Real image | Virtual image | | How the rays behave | Rays actually converge and cross | Rays diverge; only backward extensions cross | | Light at the image point | Yes, real light passes through it | No, no light reaches it | | Projection onto a screen | Yes, at the image location | No, a screen there stays blank | | Seen by the eye | Yes, directly or via the eye forming its own retinal image | Yes, by looking into the mirror or through the lens | | Orientation in single element cases | Inverted | Upright | | Size | Larger, smaller, or same size, depending on setup | Larger, smaller, or same size, depending on setup | | Location for a lens | Opposite side from the object | Same side as the object | | Location for a mirror | In front of the mirror, with the object | Behind the mirror, opposite the object | | Formed by | Concave mirrors in most positions, converging lenses with the object beyond the focal point | Plane mirrors always, convex mirrors always, diverging lenses always, converging lenses with the object inside the focal point, concave mirrors with the object inside the focal point |

Two rows in that table deserve emphasis because they correct the most common errors. First, both kinds of image can be seen by the eye. Visibility does not separate them. Second, both kinds can be larger or smaller than the object. Size does not separate them either. Only convergence, and therefore projectability, separates them.

How to Tell by Looking, and the Limits of Looking

The screen test is the practical test. Place a white card or screen where you think the image is. If a sharp image appears on the card, the image is real. If the card stays blank while you can still see the image by looking into the mirror or through the lens, the image is virtual. Move the card back and forth to find the sharp position. A real image is sharp only near its true location. Far from that plane, it blurs, just as a projector picture blurs when the screen is too close or too far away.

Looking alone can mislead, and that is worth knowing before an exam or a laboratory session. You cannot reliably tell a real image from a virtual image just by judging how the image looks to your eye. In both cases, your eye receives diverging light and forms a real image on your retina. That is why a virtual image looks real to you, and why a real image can also be viewed directly without a screen, provided your eye is positioned to collect its light. The OpenStax answer material for this chapter makes the same point: you cannot distinguish the two simply by judging from the image perceived with your eye.

A camera adds a useful twist. You can photograph a virtual image, for example your reflection in a plane mirror, and the photograph will show it. That does not make the mirror image real. The camera lens takes the diverging rays that reach it and forms a new real image on its sensor. The photograph records that second image, not light that traveled behind the mirror. The same reasoning explains why you can see a virtual image at all. Your eye is a second optical system that converts diverging input light into a real image on your retina.

Plane Mirrors: Always Virtual

A plane mirror, a flat mirror, always forms a virtual image of a real object. The image is upright, the same size as the object, and located as far behind the mirror as the object is in front of it. If you stand 2 meters from a flat mirror, your image appears 2 meters behind the mirror, and the total apparent distance from you to your image is 4 meters.

The ray picture explains why. Two rays from one point on the object reflect from the mirror by the law of reflection, which states that the angle of incidence equals the angle of reflection. Extended backward behind the mirror, those reflected rays meet at the image point. No reflected ray goes behind the mirror, so the image cannot be projected there. Repeating the construction for every point on the object gives an upright, same size image behind the mirror.

Plane mirror images also show front to back reversal, which is often misdescribed as left to right reversal. When you face a mirror, your front faces the mirror, and the image faces you, so front and back are reversed. Your right hand appears as the left hand of the image because the image is turned to face you, in the same way that a right hand glove turned inside out becomes a left hand glove. The mirror does not flip up and down, and it does not separately flip left and right. It reverses the direction perpendicular to its surface.

Multiple mirrors extend the same idea. An object between two parallel mirrors, placed at a point other than the midpoint between the mirrors, can produce a chain of images, each image acting as an object for the next reflection. In theory the chain continues without end, although in practice each reflection loses some light, so later images grow dimmer. Two mirrors at right angles produce three images of one object: one in each mirror, and one formed by reflection in both mirrors in turn. Every one of those images is still virtual, because in each case the light only appears to come from behind a mirror.

Concave Mirrors: Real or Virtual, Depending on Object Position

A concave mirror curves inward, like the inside of a bowl. It is a converging mirror. Parallel rays striking it are reflected toward a focal point in front of the mirror, under the small angle approximation used in school and introductory university treatments. Whether the image is real or virtual depends on where the object sits relative to the focal point and the center of curvature.

When the object is beyond the focal point, a concave mirror forms a real, inverted image in front of the mirror. The size depends on position. An object beyond the center of curvature gives a smaller real image between the focal point and the center of curvature. An object at the center of curvature gives a same size real image at the same place. An object between the focal point and the center of curvature gives an enlarged real image beyond the center of curvature. That enlarged real image is formed beyond the center of curvature, where real light is actually brought to a focus.

When the object moves inside the focal point, between the mirror and its focal point, the reflected rays spread apart instead of converging. Their backward extensions meet behind the mirror, giving a virtual, upright, magnified image. That is the shaving mirror and makeup mirror case. You stand close to the concave mirror, inside its focal length, and you see a larger upright face. Move back beyond the focal point, and the image flips to real and inverted, which is why the same mirror can look confusing if you step back while using it.

The full step by step ray construction for a concave mirror, including the four principal rays, is covered in our companion article on concave mirror ray diagrams. The present article keeps the rule short: outside the focal point, real and inverted; inside the focal point, virtual and upright.

Convex Mirrors: Always Virtual

A convex mirror bulges outward toward the incoming light, like the outside of a ball. It is a diverging mirror. Parallel rays striking it are reflected so that they spread apart, and they only appear to come from a focal point behind the mirror. That focal point is virtual, because no real rays pass through it.

A convex mirror always forms a virtual, upright, diminished image of a real object, located between the mirror and its virtual focal point. That combination explains its everyday uses. A shop security mirror or a vehicle side mirror shows a smaller upright image, so a larger scene fits into a small mirror. The trade off is that objects look farther away than they are, because a smaller image is what the visual system associates with greater distance. The mirror has not moved the object. It has only changed the size of the image your eye receives.

Because the image is always virtual, it can never be projected onto a screen behind the mirror, and it can never be caught on a card in front of the mirror either, because the reflected rays in front of the mirror are diverging, not converging to that image point. The detailed ray construction for this case is covered in our companion article on convex mirror ray diagrams.

Converging Lenses: Real or Virtual, Depending on Object Position

A converging lens bends light toward its optical axis. A convex lens in air is the standard example. Parallel rays entering it meet at a real focal point on the opposite side of the lens. Whether the image of a nearby object is real or virtual again depends on object position relative to the focal point.

When the object is beyond the focal point, a converging lens forms a real, inverted image on the opposite side of the lens. An object far away gives a small real image near the focal point. An object at twice the focal length gives a same size real image at twice the focal length on the other side. An object between one and two focal lengths away gives an enlarged real image farther out. Cameras and projectors use this real image regime. A projector throws an enlarged real image onto a screen. A camera forms a real image, usually diminished, on film or a sensor.

When the object is inside the focal length, a converging lens forms a virtual, upright, magnified image on the same side of the lens as the object. That is the magnifying glass case. The rays leaving the lens diverge, and their backward extensions meet on the object side, where your eye places the enlarged image. You cannot project that magnified image onto a screen on the object side, because no light converges there.

At exactly the focal point, the outgoing rays are parallel, and neither a finite real image nor a finite virtual image forms in the thin lens picture. The image is said to be at infinity. In practice, a screen placed far away shows a blurred patch rather than a sharp image, and a small movement of the object either way restores a finite image, real on one side of the boundary and virtual on the other. The focal point is therefore a boundary, not just another object position.

The shape, focal behavior, and full image cases for converging and diverging lenses are compared in detail in our companion article on converging versus diverging lenses. That article is the place to go for the lens by lens comparison. The rule needed here is shorter: a converging lens can make either kind of image, and object position decides which one.

Diverging Lenses: Always Virtual

A diverging lens bends light away from its optical axis. A concave lens in air is the standard example. Parallel rays entering it spread apart, and they only appear to come from a virtual focal point on the same side as the incoming light. A diverging lens used alone with a real object in front of it always forms a virtual, upright, diminished image on the same side as the object.

That always virtual result is why a diverging lens alone can never project an image onto a screen. It is also why diverging lenses are useful as correctors rather than as projectors. In vision correction, a diverging lens is associated with nearsightedness, where the eye system overconverges light for distant objects, because the diverging lens adds negative power and moves the effective focus back to a usable place. A converging lens is associated with farsightedness, where the eye system underconverges for near objects, because it adds positive power. Those are general optical principles, not personal eye care advice, and an eye examination is the route to an actual prescription.

Diverging lenses also appear inside multi element instruments, where the simple always virtual rule applies to that lens acting alone, but not necessarily to the system as a whole. In a compound microscope or telescope, the image formed by one element becomes the object for the next, so the final image type must be worked out step by step. The single lens rule remains true at each step. It is the system result that needs the extra care.

Reading a Ray Diagram Without Guessing

A ray diagram is a method, not a picture to memorize. For a single mirror or lens, two rays from one object point are enough to locate its image point, although tracing a third ray is a good check. The useful rays are chosen because their paths are easy to construct.

For a mirror, a ray parallel to the optical axis reflects through the focal point for a concave mirror, or along a line whose backward extension passes through the virtual focal point for a convex mirror. A ray through the focal point reflects parallel to the axis. A ray through the center of curvature strikes the mirror at normal incidence and reflects back along itself. A ray striking the vertex reflects symmetrically about the axis.

For a thin lens, a ray parallel to the axis passes through the focal point on the far side for a converging lens, or leaves along a line whose backward extension passes through the virtual focal point on the near side for a diverging lens. A ray through the center of the lens continues without deviation in the thin lens picture. A ray through the near focal point leaves parallel to the axis for a converging lens, with the corresponding construction for a diverging lens.

Once two such rays are drawn, find their meeting point. If solid rays cross, the image point is real. If only dashed backward extensions cross, the image point is virtual. If the rays are parallel after the element, the image is at infinity, and no finite image point exists in that construction. That reading rule works for mirrors and for lenses, and it is the same rule whether the diagram is drawn for a plane mirror, a curved mirror, or a lens.

Two diagram habits cause most errors. The first is drawing the dashed extensions on the wrong side. Backward extensions always go opposite to the direction of travel of the outgoing rays. For a mirror, that means behind the mirror. For a lens, that means back toward the object side. The second is tracing rays from the wrong object point, for example from the base of an arrow instead of its tip, and then reading orientation from a single point. Locate the tip image first, then note that the base of the image lies on the optical axis, and orientation follows from those two points.

The Equations, and What Their Signs Mean

Ray diagrams give type, orientation, and approximate location. Equations give numbers. For a spherical mirror in the small angle approximation, the mirror equation is 1 divided by object distance plus 1 divided by image distance equals 1 divided by focal length. For a thin lens, the thin lens equation has the same form, with distances measured from the center of the lens. In both cases, magnification equals negative image distance divided by object distance. A negative magnification means the image is inverted relative to the object. A positive magnification means it is upright. The absolute value of magnification gives the size ratio. A magnification of 2 means twice as tall. A magnification of 0.5 means half as tall.

Sign conventions must be stated before numbers mean anything, because different textbooks choose different conventions. The convention used here follows OpenStax University Physics Volume 3. For mirrors, focal length is positive for concave mirrors and negative for convex mirrors. Image distance is positive for a real image and negative for a virtual image. For lenses, focal length is positive for a converging lens and negative for a diverging lens, and the same image distance rule applies in the corresponding lens convention: a positive image distance denotes a real image on the opposite side, and a negative image distance denotes a virtual image on the same side as the object.

That sign rule gives a second, algebraic test that matches the ray test. Solve the equation. If the image distance comes out positive in the mirror convention, the image is real. If it comes out negative, the image is virtual. The sign of magnification then confirms orientation. Students who get a negative image distance and still describe the image as real have usually mixed two sign conventions, or have forgotten that a negative image distance is the algebra telling them the image is virtual.

The focal length of a spherical mirror is half its radius of curvature, under the small angle approximation. That relation connects mirror shape to mirror power. A mirror with a smaller radius of curvature curves more sharply, has a shorter focal length, and brings parallel rays to a focus closer to the mirror. These equations assume rays close to the axis and small angles, the paraxial approximation. Real mirrors and lenses used at large angles show aberrations, where rays do not all meet at one point, so real images blur. That limitation does not change the real versus virtual definition. It only limits how sharp a real image can be in a simple device.

Worked Examples

Worked Example 1: a concave mirror forming a real image. A concave mirror has a focal length of 20 centimeters. An object is placed 30 centimeters in front of it, beyond the focal point. The mirror equation gives 1 divided by image distance equals 1 divided by 20 minus 1 divided by 30, which equals 1 divided by 60. The image distance is therefore 60 centimeters, positive, so the image is real and in front of the mirror. Magnification equals negative 60 divided by 30, which equals negative 2. The image is inverted and twice as tall as the object. A screen placed 60 centimeters in front of the mirror would show it sharply.

Worked Example 2: a converging lens forming a virtual image. A converging lens has a focal length of 10 centimeters. An object is placed 5 centimeters in front of it, inside the focal length, as in a magnifier. The thin lens equation gives 1 divided by image distance equals 1 divided by 10 minus 1 divided by 5, which equals negative 1 divided by 10. The image distance is therefore negative 10 centimeters, so the image is virtual and on the same side as the object, 10 centimeters from the lens. Magnification equals negative of negative 10 divided by 5, which equals positive 2. The image is upright and twice as tall as the object. A screen placed 10 centimeters on the object side would stay blank, even though the magnified image is clearly visible through the lens.

Worked Example 3: a diverging lens forming a virtual image. A diverging lens has a focal length of negative 15 centimeters. An object is placed 30 centimeters in front of it. The thin lens equation gives 1 divided by image distance equals negative 1 divided by 15 minus 1 divided by 30, which equals negative 1 divided by 10. The image distance is therefore negative 10 centimeters, so the image is virtual and on the same side as the object. Magnification equals negative of negative 10 divided by 30, which equals positive one third. The image is upright and one third as tall as the object. No position of a real object in front of this lens alone will change that virtual result. Moving the object closer or farther changes the size and location, but the image stays virtual, upright, and diminished.

Notice what the three examples have in common. In each case, the ray test and the sign test agree. Positive image distance goes with real and inverted. Negative image distance goes with virtual and upright. When a calculation and a diagram disagree, check the sign convention first, then check whether the object is inside or outside the focal point, because that boundary is where the image type flips for converging devices.

Everyday Examples, Sorted Correctly

Sorting everyday examples by the convergence test clears up several half remembered claims.

A cinema projector, a slide projector, and a camera all use real images. The projector forms a real image on a screen. The camera forms a real image on film or a sensor. A 35 millimeter film camera is therefore a real image device at the recording plane, even though the photographer looks through a viewfinder system that may itself present a virtual view for framing.

Your eye forms a real image on your retina. Light entering the eye is converged by the eye lens system to a focus on the retina, where light sensitive cells respond. Cone receptors in the eye are part of that detection story, but the image formation step itself is a real image step. That is also why the earlier point about seeing a virtual image makes sense. When you look at a mirror, the mirror image is virtual, but your eye then forms a real image of that diverging light on your retina. Two steps are involved, and they must not be confused.

A bathroom mirror, a magnifying glass held close to print, a shop security mirror, and a vehicle side mirror all present virtual images to the viewer. The bathroom mirror image is same size and upright. The magnifier image is enlarged and upright. The security and side mirror images are diminished and upright. All four are seen by looking into the device, and none of them can be projected onto a screen placed at the apparent image location.

A pencil in a glass of water shows a virtual image formed by refraction, not by a mirror or a lens in the usual sense. Light from the submerged part of the pencil bends at the water surface, so the rays reaching your eye appear to come from a shifted position. The pencil looks bent or broken at the surface because different parts of the pencil are seen through different amounts of refraction. Refraction explains the bending itself, and the index of refraction of water sets how strong that bending is for water compared with air. The image you see is virtual, because the light reaching your eye does not actually come from the apparent position of the submerged pencil.

A rainbow is not a good example for this article, and it is often misused as one. A rainbow is not an image formed by a single mirror or lens, and it does not have a single image point that a screen could catch in the same way. It is a pattern formed by refraction and reflection in many droplets, seen in different directions by different observers. Leaving it out keeps the real versus virtual test clean. The test applies to image formation by an optical element, not to every bright pattern in the sky.

Five Mistakes to Avoid

Mistake 1: thinking a virtual image is not really there, or that it is a trick. A virtual image has a definite apparent location, a definite size, and a definite orientation. Your eye measures it consistently. What it lacks is light at that location. Absence of light there, not absence of an image, is the point.

Mistake 2: thinking a real image must be on a screen to exist. The rays cross whether or not a screen is present. A screen reveals the crossing. It does not create it. A real image in air can be viewed directly, and in a darkened room with dust or smoke in the beam, the converging cone of light can sometimes be seen approaching its focus.

Mistake 3: thinking inverted means real and upright means virtual in every system. That pattern holds for the single element cases in this article, but in a multi element system the image of one element becomes the object for the next, and the final orientation is the product of the steps. A telescope or microscope can present an upright final view built from inverted intermediate images. The convergence test at each step is still reliable. The orientation shortcut is not.

Mistake 4: thinking a virtual image cannot be photographed or recorded. It can, because a camera or an eye forms a new real image from the diverging rays. The photograph is a record of the light that reached the camera, not proof that light traveled behind the mirror. The same point applies to a video of a mirror reflection. The recording device did the converging.

Mistake 5: thinking the side rule is the same for mirrors and lenses. For a lens, real means opposite side and virtual means same side. For a mirror, real means front side, with the object, and virtual means behind the mirror. Memorizing one side rule and applying it to both devices guarantees errors. Memorize the convergence rule instead, and derive the side from it for each device.

When the Simple Rules Stop Applying

The rules in this article assume a single thin lens or a spherical mirror, rays close to the optical axis, and a real object in front of the device. Real optical work often breaks one of those assumptions, and the article should say so rather than pretend the simple rules cover everything.

Thick lenses bend light at two separated surfaces, so the thin lens picture is an approximation. It remains useful, but exact locations need the fuller treatment. Large angle rays do not all meet at one focal point for a spherical mirror or a simple lens, an effect called spherical aberration. Different colors focus at different points, an effect called chromatic aberration, because refractive index depends on wavelength. Both effects blur a real image. They do not turn it into a virtual image.

Multi element systems need step by step analysis. The image formed by the first element is the object for the second element. That intermediate object can be real or virtual, depending on where the first image falls relative to the second element. A virtual object is a point toward which light was converging before a second element intercepted it. No light reaches that point, because the second element redirects the rays first, but the point is still the correct starting point for the second step of the calculation. The language of real and virtual therefore applies to objects as well as to images in fuller treatments, with the same convergence logic: light diverges from a real object or a virtual image, and light converges to a real image or a virtual object.

Finally, the geometric optics picture itself assumes light travels in straight lines except at surfaces, and that wavelength effects can be ignored. For very small structures, diffraction and wave effects matter, and the sharp ray picture breaks down. That is beyond the scope of a first comparison article, but it is the reason advanced optics does not stop at ray diagrams.

FAQ

What is the difference between a real image and a virtual image?

A real image forms where light rays actually converge, so it can be projected onto a screen. A virtual image forms where rays only appear to converge when traced backward, so it can be seen by looking into the mirror or through the lens but cannot be projected onto a screen. In single mirror and single lens cases, real images are inverted and virtual images are upright, but convergence is the definition.

Can a virtual image be projected onto a screen?

No. A screen placed at the apparent location of a virtual image stays blank, because no light travels to that point. You can still see the virtual image, and you can photograph it, because your eye or a camera forms a new real image from the diverging rays that reach it. That new image is not the virtual image projected. It is a second image formed by a second optical system.

Can a real image be seen without a screen?

Yes. A real image exists at the point where rays cross, whether or not a screen is there. You can view it directly if your eye is positioned to collect its light, because your eye then forms its own real image on your retina. A screen is a convenient way to reveal a real image, not a condition for its existence.

Is the image in a plane mirror real or virtual?

Virtual. A plane mirror always forms a virtual image of a real object. The image is upright, the same size as the object, and as far behind the mirror as the object is in front of it. It cannot be projected onto a screen behind the mirror, because reflected light never goes there.

Which mirrors and lenses always form virtual images?

A plane mirror always forms a virtual image. A convex mirror always forms a virtual image. A diverging lens used alone with a real object always forms a virtual image. Those three always virtual cases are the safe answers when a question asks for a device that cannot project an image on its own.

Which mirrors and lenses can form either kind of image?

A concave mirror and a converging lens can each form either a real image or a virtual image, depending on object position. When the object is beyond the focal point, the image is real and inverted. When the object is inside the focal point, the image is virtual and upright. At the focal point itself, the outgoing rays are parallel, and no finite image forms in the thin lens and small angle picture.

Why does a convex mirror make things look farther away?

A convex mirror forms a diminished virtual image. Because the image is smaller than the object, the visual system tends to judge the object as farther away than it is. The object itself has not moved. The mirror has only changed the size of the image presented to the eye, while also fitting a wider scene into a small mirror.

How do I use the mirror equation to check whether an image is real or virtual?

Write the mirror equation with a consistent sign convention, solve for image distance, and read its sign. In the OpenStax convention used here, a positive image distance means a real image and a negative image distance means a virtual image. Then check magnification. Negative magnification means inverted, which should go with a real image in these single element cases, and positive magnification means upright, which should go with a virtual image. If the signs disagree with your ray diagram, recheck the sign convention before changing the diagram.

Does a virtual image mean the image is fake?

No. Virtual describes where the light actually travels, not whether the image is genuine. A virtual image has a consistent apparent position, size, and orientation, and different observers looking into the same mirror or lens see consistent images from their own viewpoints. Fake would mean the image does not follow the laws of reflection or refraction. A virtual image follows them exactly. It is simply located where the light appears to come from, rather than where the light actually goes.

Sources & references

• OpenStax, University Physics Volume 3, Section 2.1 Images Formed by Plane Mirrors. https://openstax.org/books/university-physics-volume-3/pages/2-1-images-formed-by-plane-mirrors
• OpenStax, University Physics Volume 3, Section 2.2 Spherical Mirrors. https://openstax.org/books/university-physics-volume-3/pages/2-2-spherical-mirrors
• OpenStax, University Physics Volume 3, Section 2.4 Thin Lenses. https://openstax.org/books/university-physics-volume-3/pages/2-4-thin-lenses
• OpenStax, University Physics Volume 3, Chapter 2 Summary. https://openstax.org/books/university-physics-volume-3/pages/2-summary
• Physics LibreTexts, Geometric Optics by Jeremy Tatum (University of Victoria), Section 2.3 Real and Virtual. https://phys.libretexts.org/Bookshelves/Optics/Geometric_Optics_(Tatum)/02:_Lens_and_Mirror_Calculations/2.03:_Real_and_Virtual
• OpenStax, University Physics Volume 3, Answer Key Chapter 2, Conceptual Questions 1, 3, and 5. https://openstax.org/books/university-physics-volume-3/pages/chapter-2