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I'm doing magnification and lens in class currently, and I really don't get why virtual and real images are called what they are.

A virtual image occurs the object is less than the focal length of the lens from the lens, and a real image occurs when an object is further than focal length.

By why virtual and real? What's the difference? You can't touch an image no matter what it's called, because it's just light.

Qmechanic
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Jonathan.
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4 Answers4

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You can project a real image onto a screen or wall, and everybody in the room can look at it. A virtual image can only be seen by looking into the optics and can not be projected.

As a concrete example, you can project a view of the other side of the room using a convex lens, and can not do so with a concave lens.

I'll steal some image from Wikipedia to help here:

First consider the line optics of real images (from http://en.wikipedia.org/wiki/Real_image):

real images formed by a single convex lens of concave mirror

Notice that the lines that converge to form the image point are all drawn solid. This means that there are actual rays, composed of photon originating at the source objects. If you put a screen in the focal plane, light reflected from the object will converge on the screen and you'll get a luminous image (as in a cinema or a overhead projector).

Next examine the situation for virtual images (from http://en.wikipedia.org/wiki/Virtual_image):

virtual images formed by a single concave lens or convex mirror

Notice here that the image is formed by a one or more dashed lines (possibly with some solid lines). The dashed lines are draw off the back of solid lines and represent the apparent path of light rays from the image to the optical surface, but no light from the object ever moves along those paths. This light energy from the object is dispersed, not collected and can not be projected onto a screen. There is still a "image" there, because those dispersed rays all appear to be coming from the image. Thus, a suitable detector (like your eye) can "see" the image, but it can not be projected onto a screen.

dmckee --- ex-moderator kitten
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    Thanks for the answer, it helps quite a but, could you maybe expand on it, like why you can't project a virtual images? – Jonathan. Jan 09 '11 at 22:32
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    This answer doesn't make the image clear to me. (No pun intended!) – Noldorin Jan 09 '11 at 22:34
  • Better with the (stolen) images... I still thing this answer rather misses the point though. – Noldorin Jan 10 '11 at 02:00
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    On the contrary, I think this answer explains the physical picture a lot better. – ptomato Jan 11 '11 at 22:25
  • I knew the answer to this question, but when I thought about it, I stumbled into another one; I believe eye also act like a screen, then why is that a virtual image can be seen with eyes?? – Vineet Menon Mar 29 '12 at 04:51
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    @VineetMenon : Because your eye contains a lens, which itselfs forms a real image on the retina. – Frédéric Grosshans Jun 13 '12 at 17:45
  • If our eyes can see the virtual images, can our eyes also see the real images? – kiss my armpit May 06 '14 at 16:29
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    @IamwhoIsayIam When the rays pass through the real image they are diverging again and it is exactly the same as seeing a virtual image. So, short answer: yes. – dmckee --- ex-moderator kitten May 06 '14 at 19:06
  • I am having a confusion as to what "converge" mean right now? But I guess it means intersect. You mentioned how virtual image can be projected by means of two or more light intersecting. Does this mean you have to place the screen right at where those lines intersect? Any closer or farther I would not get a clear image? – most venerable sir Aug 01 '15 at 00:11
  • I think this how a telescope work. I guess there is a screen in the telescope, positioned at the focal point of a concave mirror. The light rays, which are essentially parallel, coming from a faraway star get converged at the focal point of that concave mirror in that telescope. (Only a guess) – most venerable sir Aug 01 '15 at 00:33
  • @Doeser You can put a screen in front of real image, but you don't have to. You can simply look at them as the diverge again after passing though one another. A telescope has two (or more) lenses and no screen if you are going to look through it: you just view the light. – dmckee --- ex-moderator kitten Aug 01 '15 at 04:44
  • But the light rays must intersect the moment they reach the screen?? If they don't the image will be out of focus? – most venerable sir Aug 01 '15 at 11:50
  • @Doeser Some possibly useful links: http://physics.stackexchange.com/questions/6483/why-can-you-see-virtual-images http://physics.stackexchange.com/questions/14688/focal-point-vs-where-you-see-the-images http://physics.stackexchange.com/questions/192467/in-a-lens-the-image-formed-by-the-first-surface-serves-as-the-object-for-the-sec – dmckee --- ex-moderator kitten Aug 01 '15 at 16:21
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Operationally, one can distinguish real and virtual images by attaching a small but very powerful light bulb to the source.

If you insert your hand near the real image, the hand will get burned because the light rays from the source actually converge at the point of the real image. If you manage to insert your hand to the point of the virtual image, your hand won't burn because there are no light rays over there (and sometimes, there is a wall).

An ordinary flat mirror creates a virtual image because there are no light rays on the internal side of the mirror.

The optics in the human eye creates a real image where the retina is located and sensitive cells actually read the information about the original source. The same mechanism works in digital cameras.

Real images may be produced by converging lenses (eye, digital camera) and concave mirrors.

Virtual images may be produced by diverging lenses and convex mirrors, but also by concave mirrors and converging lenses if you place the source within the focal length.

Luboš Motl
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For a real image, rays from a single source point converge to a single point on the other side of the lens. This means that a point on the image remains well-defined after the optical transformation (refraction by lens).

For a virtual image, rays from a single source point diverge after they pass through the lens. This means that a point on the image is no longer well-defined after the optical transformation. Realistically, what you'll see on a screen place at the other end of a diverging (concave) lens is a large haze of weak light (since the rays have diverged so greatly).

Note that convex (converging) single lenses always produce real images, while concave (diverging) single lenses always produce virtual images. For mirrors it's generally the other way round (there are exceptions though). Similarly for double lenses, things are reversed (compared to single lenses).

Noldorin
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  • So a magnifying glass for example is that a real or virtual image? – Jonathan. Jan 09 '11 at 23:02
  • That creates a virtual image actually. I should clarify that point in the answer - I only mean single lenses there (not double lenses like magnifying glasses). – Noldorin Jan 09 '11 at 23:56
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    You got things mixed up on the last paragraph. A convex mirror always produces virtual images, but a concave mirror can produce both. I think the same (but reversed) happens for lenses. http://en.wikipedia.org/wiki/Curved_mirror – Malabarba Jan 10 '11 at 01:00
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    A magnifying glass is not just one lens?? That explains a lot :) – Jonathan. Jan 10 '11 at 01:09
  • @Bruce: Read again, I think you'll find you got things muddled. I quote: "For mirrors it's precisely the other way round.". Please don't be so quick to down-vote. – Noldorin Jan 10 '11 at 01:59
  • @Jonathan: Exactly. It's a double lens (both sides are curved). When we usually talk of lenses, we in fact mean double rather than single lenses... – Noldorin Jan 10 '11 at 02:01
  • @Jonathan: Take note that Noldorin is using the jargon of lens making where "single" and "double" refer to how many sides of the lens have been ground. You can make a magnifying glass from either as long as the lens is convex (or the convex side is stronger than the concave side in the case of a mixed lens. That said, Magnifying class can form real images, as that is how you burn leaves with them: image the surface of the sun onto the target and the heat does the trick. – dmckee --- ex-moderator kitten Jan 10 '11 at 02:34
  • @Noldorin: I down voted because it seems to me the answer has a wrong statement. If I turn out to be wrong, I'll be happy to remove it (and up vote), it's a pretty good question other than that. The thing is that concave mirrors can produce both types of images (real and virtual), but the way your worded it seems your saying they can only produce real images. If that really is what you are saying, then it is not accurate. If that is not what you're saying, then clarify and I'll be glad to up vote. :-) – Malabarba Jan 10 '11 at 17:14
  • @Brace: Ok, now you've explained yourself, that makes a lot more sense. I'll clarify the answer. :) – Noldorin Jan 10 '11 at 18:15
  • @Noldorin: There we go. Though I'm not sure about the lenses either. I was positive that one of them can produce both types of image as well, but I could be wrong now. – Malabarba Jan 10 '11 at 19:29
  • @Bruce: You've made me unsure now too! ;) If I saw a diagram, I would probably be convinced quite quickly, but I'll keep it how it is for now hah. – Noldorin Jan 10 '11 at 20:13
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What we mean by a real image is that rays of light emanating from a given point on an object are reflected or refracted so that they converge at a second point to form an image. The word 'real' means that the rays of light actually converge to form the image. You can put a screen where they converge and the image will appear on the screen.

A virtual image arises where light is reflected or refracted in such a way that it only appears to have converged to form an image- no convergence has actually taken place.

Marco Ocram
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