Collection of focus points made by converging light rays
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In optics, an image is defined as the collection of focus points of light rays coming from an object. A real image is the collection of focus points actually made by converging/diverging rays, while a virtual image is the collection of focus points made by extensions of diverging or converging rays. In other words, it is an image which is located in the plane of convergence for the light rays that originate from a given object. Examples of real images include the image produced on a detector in the rear of a camera, and the image produced on an eyeball retina (the camera and eye focus light through an internal convex lens).
In ray diagrams (such as the images on the right), real rays of light are always represented by full, solid lines; perceived or extrapolated rays of light are represented by dashed lines. A real image occurs where rays converge, whereas a virtual image occurs where rays only appear to diverge.
Real images can be produced by concave mirrors and converging lenses, only if the object is placed further away from the mirror/lens than the focal point, and this real image is inverted. As the object approaches the focal point the image approaches infinity, and when the object passes the focal point the image becomes virtual and is not inverted (upright image). The distance is not the same as from the object to the lenses.
Real images may also be inspected by a second lens or lens system. This is the mechanism used by telescopes, binoculars and light microscopes. The objective lens gathers the light from the object and projects a real image within the structure of the optical instrument. A second lens or system of lenses, the eyepiece, then projects a second real image onto the retina of the eye.
See also
- Focal plane
- Image plane
- Lens
- Virtual image
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Is there a simple way to determine or prove this? Real images are always inverted, and unlike virtual images can be projected onto a screen (I'm not even sure what this means to be honest). If I look at the back end of a spoon (convex mirror), the image is always upright and therefore virtual, in back of the mirror. Makes sense, since I'm in front of the mirror. Then I turn to the concave side of the spoon and keep it less than one focal length away from me, still the image is virtual and upright, and supposedly still in back of the mirror. But after moving the mirror more than a focal length away from me the image is now inverted, and therefore real. Does this mean it's in front of the mirror? I can't tell the difference. Please clarify and correct any misstatements/misassumptions.
Answers and Replies
sophiecentaur
Real images are always inverted, and unlike virtual images can be projected onto a screen
Nowadays, kids are often given laser pointers for this sort of experiment but they actually fail to beat the Pin method when you need understanding.
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Is there a simple way to determine or prove this? Real images are always inverted, and unlike virtual images can be projected onto a screen (I'm not even sure what this means to be honest). If I look at the back end of a spoon (convex mirror), the image is always upright and therefore virtual, in back of the mirror. Makes sense, since I'm in front of the mirror. Then I turn to the concave side of the spoon and keep it less than one focal length away from me, still the image is virtual and upright, and supposedly still in back of the mirror. But after moving the mirror more than a focal length away from me the image is now inverted, and therefore real. Does this mean it's in front of the mirror? I can't tell the difference. Please clarify and correct any misstatements/misassumptions.
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Merlin3189
... why you cannot tell the difference between the real and the virtual image formed by the concave mirror. .... Slightly divergent rays coming from the tip of the arrow are bent by the lens of your eye and focused on your retina.
... Your brain has learned to extend nearly parallel but divergent rays to their point of intersection in front of you and that's where you "see" the image to be. .
** - It's not because I have the telescope in normal adjustment - I'm myopic, so I need an image about 50 cm away.
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How can your brain know what direction individual rays are coming from, if they are all focused to a single spot?
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Merlin3189
Sorry, I thought you were talking about all the rays from a single point (or single star) diverging to your pupil and then being converged to a single point on your retina. I shall have to reread your post.
Individual rays stimulate individual spots on the retina ....
As far as that goes, I don't think where the image is nor what it looks like is important. Simply whether the light passes through the image, or would do so in the absence of other obstructions (including such as a second mirror or lens.) An objective criterion rather than a subjective one.
Likes Fizzizist
Is there a simple way to determine or prove this? Real images are always inverted, and unlike virtual images can be projected onto a screen (I'm not even sure what this means to be honest). If I look at the back end of a spoon (convex mirror), the image is always upright and therefore virtual, in back of the mirror. Makes sense, since I'm in front of the mirror. Then I turn to the concave side of the spoon and keep it less than one focal length away from me, still the image is virtual and upright, and supposedly still in back of the mirror. But after moving the mirror more than a focal length away from me the image is now inverted, and therefore real. Does this mean it's in front of the mirror? I can't tell the difference. Please clarify and correct any misstatements/misassumptions.
Zz.
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I suppose, if it were dim enough to emit a single photon at a time, or if your pupil were small enough (& ignoring diffraction), then maybe we could say there was only a single ray from each point on the object to each point on the image.
I agree.
But it's not worth discussing further, as it relates to visual perception rather than real and virtual images that op asked about.
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Great info everyone, thanks. So the eye can't tell the difference. A movie is a real image, while a mirage is virtual. A plane mirror is always upright and therefore virtual: the source of light is in front of the mirror but it appears in back. Does the curvature reflect the light more than once in the concave mirror, changing the source so it's in back? Somewhat like the reason letters appear backwards in a mirror?
Mister T
Real images are always inverted, and unlike virtual images can be projected onto a screen (I'm not even sure what this means to be honest).
Make the ray-tracing diagrams. When the rays "really" converge to form an image, that's a real image. When they don't, it isn't.
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Mister T
If I look at the back end of a spoon (convex mirror), the image is always upright and therefore virtual,[...]
I don't follow. The image is behind the mirror and you are in front of the mirror. But I don't understand what connection between the two leads you to conclude that that makes sense. It does make sense, but I wonder why it makes sense to you. The reason it makes sense to me is that when I draw the ray diagram I see that the rays appear to have originated from an image, and the place where that happens is behind the mirror.
[...] in back of the mirror. Makes sense, since I'm in front of the mirror.
No supposition involved. Draw the ray diagram and see for yourself that the rays appear to come from an image, that that image is indeed behind the mirror and that that image is upright.
Then I turn to the concave side of the spoon and keep it less than one focal length away from me, still the image is virtual and upright, and supposedly still in back of the mirror.
But after moving the mirror more than a focal length away from me the image is now inverted, and therefore real.
Being inverted is not the cause of it being real. What makes it real is that the rays "really" do converge to form an image. And when you draw the ray diagram you do indeed see that the image is inverted.
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Try with a concave mirror and you sometimes can.
Last edited: May 10, 2018
Likes Fizzizist
Try with a concave mirror and you sometimes can.
I don't either, it's just something to accept without proof and hope everything reconciles later on. A common but suboptimal learning technique :O
The image is always upright and the image is always virtual, but I don't see how either one of those is the cause of the other.
How do I draw the ray diagram? Measure the object and image distances from the mirror, and also that of the focal length? Is it even possible to measure the image distance without a sensor/computer?
Mister T
How do I draw the ray diagram?
The task itself is not particularly hard, although it does take a lot of practice, but it is hard to explain it in a post. The time spent learning this is time well spent because it helps you understand what the equations are telling you.
sophiecentaur
How do I draw the ray diagram?
As @Mister T says, you can find loads of sources that will show you what to do and give you examples to solve - which will show you how well you have understood what they tell you.
Googling is nice, but I like direct answers to questions (which you all have been kindly providing!) This isn't homework, just me trying to understand the concepts better.