# What's the difference between a virtual and a real image

## Main Question or Discussion Point

I cannot seem to get the concept of real and virtual image. This is my understanding.
If the light beams go to the reflecting surface(concave or convex mirror) then an image will be form assuming the reflecting beams meat at a point. If the image appears to be in front of the mirror(s' is positive) then the image is real. If the image appears to be behind the mirror(s' is negative) the image is virtual. Am I correct on this?

When reading an example from my textbook about image formation by refraction. A light coming from air and going into a convex surface(left to right). s' ended up being positive and to the right of the refracting surface. Shouldn't the image be on the same side as the actual object since s' is positive?
Some help would be appreciated.

## Answers and Replies

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Andy Resnick
Science Advisor
Education Advisor
Real images are inverted and located in 'image space': placing a screen or detector at the image plane will show the image. Virtual images are upright and located in 'object space': on the 'wrong side' of the optical system. Virtual images are projected into image space by the optical system.

For example, your eye makes a real image- the retina is physically located at the image plane. Eyepieces generate a virtual image (ideally) at negative infinity: when looking through an eyepiece, the image is in focus when your eye is relaxed and focused at infinity.

sophiecentaur
Science Advisor
Gold Member
A real image is formed by the 'rays' actually passing through a point in space (a so-called focus). A virtual image only 'looks like it'. The rays coming off a convex mirror always appear to come from a point behind the surface of the mirror so that image is always virtual and smaller. The image in a plane mirror is, likeways, virtual because it appears somewere it can't actually be.
An appropriate arrangement of lenses or mirrors can produce a real or virtual image, depending on how you set them up. A concave (shaving) mirror with a large radius (almost flat) will produce a virtual image of your face if you stand close - it's magnified and appears behind the wall. If you move back, the image expands and expands until it appears to 'burst' - then, when you are far enough away beyond the focal length, you see a real image where the rays from parts of your face are actually passing through points in space (at a focus), between you and the mirror; that image is real, looks smaller and is upside down in this case.

A simple concave lens will never form a real image from an object - the rays will always spread out more and the image will appear closer than the object - but it isn't really there. HOWEVER, concave glasses are used in conjunction with your eye lens and, of course, a real image is produced on your retina. Also, a concave lens can be used to change the focus of a convex lens (many camera lenses, actually) - so again, a real image can be formed.

Basically, I'm saying that the real or imaginary description refers to the image and not to the kind of lens/ mirror system used. 'They say' that a real image is projected on a screen but it is not necessarily actually projected it just could be.

Redbelly98
Some other difference can be : The focal length of real image is positive but virtual image is negative.

sophiecentaur
Science Advisor
Gold Member
Some other difference can be : The focal length of real image is positive but virtual image is negative.
You are using the wrong terms here. Focal Length is a property of the lens and not the position of an image.
The sign of the position of an image all depends upon which sign convention you happen to be using. The first sign convention we learn (elementary optics) is not the only one that can be used. See here. Just to confuse you further -

Merlin3189
Henryk
Gold Member
Take a look at the picture. The top shows a real image formed by a single lens while the bottom part shows a virtual image.
As you can see, the real image is formed when light rays from a point of an object actually converge in a point in space. It is easy to verify that the real image is formed: just insert a screen there and you will see the real image on the screen.
If you stand behind the image and look at it you will see it at the image plane. That is, your eyes will see light diverging from the image plane.
The bottom part shows a virtual image. If you look at it, your eyes will see an image coming from the image plane but if you put a screen there you will see no image on the screen.

1.) A real image is reproduction of image via light where rays appear to converge that can be formed on a screen .
Where as A virtual image is reproduction of image via light where the light rays appear to diverge by crossing from a point , opposed to form an image from actual divergence .

2.) A real image is always inverted .
A virtual image is always erect.

3.) Real image Can be caught on a screen.
Virtual kmage cant be caught on a screen.

4.) Real image can be magnified or diminished.
Virtual images can be formed of the same size even if it is far or near by they appear to be same.

5.) Real images are formed in front of the mirror.
Virtual images are formed behind the mirror.

6.) Real images are inverted.
Virtual images formed are erect.
This is how we differenciate between real and virtual images

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sophiecentaur
Science Advisor
Gold Member
This reads more like a catechism than a statement of Understanding. I wonder whether you are applying those statements to a particular optical system????
I can suggest loads of (only sightly) complicated optical systems that do not follow all of those rules.
The only thing that distinguishes a real image without exception is that the light rays actually pass through it. The thing that distinguishes a virtual image is that they don't. Apply that simple rule and you can't go wrong.
But, really, giving an image the name "virtual" or "real" is not very helpful. It doesn't tell you how to work out anything about the way the light passes through a system. But I realise (and sympathise) that a number of elementary Science courses do make a big fuss about things like that.

Merlin3189
Merlin3189
Homework Helper
Gold Member
I've liked SophieCentaur's posts and particularly this simple definition.
A real image is formed by the 'rays' actually passing through a point in space (a so-called focus). A virtual image only 'looks like it'.
But I couldn't quite swallow the whole of that post because of the Cassegrain telescope.
The rays coming off a convex mirror always appear to come from a point behind the surface of the mirror so that image is always virtual and smaller. The image in a plane mirror is, likeways, virtual because it appears somewere it can't actually be. ...

So here, have we got a concave mirror forming a real image (which the light does not pass trough after reflection) which forms a virtual object for the convex mirror, with a virtual focal length, which forms a real image. And a Newtonian reflector would produce a real image from a plane mirror, again using a virtual object.
I think you can get any sort of image off any lens or mirror, if you do it right.
Otherwise, SC's got it nailed, even if I feel we should modify it to something like, rays converging towards a point in space that they would pass through if you didn't put something in the way!
But whatever you do, don't just rely on -ve answers being virtual, unless you already understand this and know you're using a real-is-positive sign convention.

Arshu
jtbell
Mentor
I feel we should modify it [real image] to something like, rays converging towards a point in space that they would pass through if you didn't put something in the way!
That's how I feel about it, too. (I seem to remember a thread not too long ago that also got stuck on this point.)

sophiecentaur
Science Advisor
Gold Member
I think you can get any sort of image off any lens or mirror, if you do it right.
Yes, absolutely. The "six articles" can only apply for a single optical element. Several elements can present the following element with a Virtual Object and then the rules go out of the window. (e.g. your Cassegrain system).
But this all shows the futility of losing sleep over what name to give something when the calculations deal with it perfectly well.