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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.

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Good conceptual question, actually. –  Noldorin Jan 9 '11 at 21:57
    
You can use following app to visualize real vs imegenary image. play.google.com/store/apps/details?id=com.shakti.rayoptics –  Shakti Malik Feb 21 at 6:25

3 Answers 3

up vote 19 down vote accepted

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.

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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 9 '11 at 22:32
    
This answer doesn't make the image clear to me. (No pun intended!) –  Noldorin Jan 9 '11 at 22:34
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@Kit: Bare links to outside resources are discouraged because Stack Exchange sites are meant to be resources that stand on their own, and a link plus text describing the pictures at the other end is unreliable because the pictures might change. That said, links are good for license reasons. Added. –  dmckee Jan 17 '11 at 14:42
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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
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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 May 6 at 19:06

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.

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I love you bro nice answer you should be awarded with a noble prize –  Help - I need somebody's help Oct 5 '13 at 16:16

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 the 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).

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So a magnifying glass for example is that a real or virtual image? –  Jonathan. Jan 9 '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 9 '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. en.wikipedia.org/wiki/Curved_mirror –  Bruce Connor Jan 10 '11 at 1:00
    
A magnifying glass is not just one lens?? That explains a lot :) –  Jonathan. Jan 10 '11 at 1:09
    
@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 1:59

protected by Qmechanic Mar 13 '13 at 16:15

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