# Concave mirrors mystery

## Main Question or Discussion Point

ok my doubt is wen i look at a concave mirror(behind focus of mirror) i can see image of my face inverted inside the mirror,,,why does this hapen?? acc to mirror laws i shud get a real image tht too not inside the mirror but outside it!!!
y do i observe an inverted virtual image in case of concave mirror..(u can try it out by just taknin a spoon and look at it u can find ur inverted image inside the spoon)..
wat is the reason behind it???(as far as i knw drawing ray diagrams did not lead to any conclusion)

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Doc Al
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Basically, the light is pointed inward, due to the angle of the concave surface. It travels inward until the point where the light crosses, creating an upside down image. If you get close enough to the spoon, you will be the right way around again!

i still dnt get it... how is it not explained by ray optics? n wat did u mean by "get close enough to the spoon,you ll be the right way around"?

Doc Al
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how is it not explained by ray optics? n wat did u mean by "get close enough to the spoon,you ll be the right way around"?
It is explained by ray optics. Look at the link I provided for the details. When the object is outside the focal length, the image is real and inverted; when inside the focal length, virtual and right side up.

i still dnt get it... how is it not explained by ray optics? n wat did u mean by "get close enough to the spoon,you ll be the right way around"?
And if you were able to crawl inside your spoon, i.e. be within the focal length, you will see yourself 'right-side-up'.

And if you were able to crawl inside your spoon, i.e. be within the focal length, you will see yourself 'right-side-up'.
Since a spoon is so small, you can't see this effect (unless you have really good eyes lol). Try a shaving magnefying mirror instead. If you use it like you normally would (at short distance) you are magnified and right side up. If you move away from it, you will turn upside down!

@nick89
thtz wat i exactly wanted to knw y is it tht i can see an inverted image(wh is virtual,since i can see it inside the mirrror) wel thru my ray diagrams i was only able to find out tht i ll get a real image but i dint understand why i m seeing a virtual inverted image!!!

Doc Al
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...but i dint understand why i m seeing a virtual inverted image!!!
The inverted image is a real image, not a virtual one. Take something small--like the tip of a pen--and move it towards the spoon. At first you'll see a real, inverted image, which if you look carefully enough will appear to be floating in front of the spoon (between object and spoon). When you move it close enough, the image will flip into a virtual image.

is this the reason why they use convex mirrors for security mirrors?

Doc Al
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is this the reason why they use convex mirrors for security mirrors?
Partly. Convex mirrors do form virtual images, which are always right side up and easier to interpret. But the main reason convex mirrors are used for security (and other applications, such as passenger side mirrors in cars) is because of their greater field of view.

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Could someone answer the question please? Does a concave mirror focus real rays of light to form a REAL image in front of the mirror, which can only be caught on paper AND at the same time it is seen IN the spoon as a virtual image that is inverted. All texts do not show the ray diagram for this case.

jtbell
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Does a concave mirror focus real rays of light to form a REAL image in front of the mirror, which can only be caught on paper AND at the same time it is seen IN the spoon as a virtual image that is inverted.
You do not get a virtual image and a real image at the same time, from the same object. Depending on the location of the object, you get one or the other of the following:

A. a real image in front of the mirror, which you can either capture on paper, or look at directly provided that you are far enough away from the mirror. In this case the image is inverted.

B. a virtual image behind the mirror, which you cannot capture on paper, but you can look at it directly. In this case the image is upright.

Hi JTBELL:

I'm a UofT Physics Major and now a highschool physics teacher (obviously not a very good one:). I know what the ray diagrams are suppose to look like but just like the guy/gal who started this thread, I think the question has been miscommunicated. Let's try again:

When the object is between C and F, the ray diagram puts the image underneath the PA, and it's larger and clearly REAL. If a piece of paper is put there, this REAl image will be caught. If a piece of paper is NOT placed there what would an observer, behind the object, see in the concave mirror. Everyone would agree that he would see a virtual image of the object inside the mirror, inverted and larger.

This appears to be in conflict with the 5 or so possible ray diagrams for objects between C and F.

Another way of stating this is how could the real inverted image that is formed in front of the concave mirror same side as the object, "floating" as DOC Al puts it, be viewed in the concave mirror as an inverted larger image - that's what i see:)

Thanks ALL

Doc Al
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When the object is between C and F, the ray diagram puts the image underneath the PA, and it's larger and clearly REAL. If a piece of paper is put there, this REAl image will be caught.
OK.
If a piece of paper is NOT placed there what would an observer, behind the object, see in the concave mirror.
Assuming the observer is far enough back that the image is in front of him, he'd see that real image.
Everyone would agree that he would see a virtual image of the object inside the mirror, inverted and larger.
No, why would you think that?

jtbell
Mentor
When the object is between C and F, the ray diagram puts the image underneath the PA, and it's larger and clearly REAL. If a piece of paper is put there, this REAl image will be caught.
Correct.

If a piece of paper is NOT placed there what would an observer, behind the object, see in the concave mirror. Everyone would agree that he would see a virtual image of the object inside the mirror, inverted and larger.
No, he would see the same real image, floating in front of the mirror. You can verify that the image is actually located in front of the mirror as follows.

First put the screen in place and focus the image. Position a pencil or other pointer-like object so it's touching the image. Remove the screen without moving the pencil. It helps if you have the pencil mounted on a stand or support so you can keep it fixed in the same position. Now look into the mirror so you see the image and the pencil simultaneously (the actual pencil, not its image in the mirror). As you move your head from side to side, the image appears to be "stuck" to the pencil. Furthermore, if you're close enough to the image that your eyes' 3-D depth perception works, you can actually see that the image is floating in front of the mirror because your eyes will focus on it at that location.

sophiecentaur
Gold Member
If you want to prove to yourself whether or not the image in front of the mirror is real, use some tissue paper just off axis and you will find that you can focus an image of the Sun on it. If it focuses, it's real.

Re: Doc Al's comment ...why do you think its virtual?

Thanks Doc and JT for the response.

When you draw the ray diagram for an object between C and F, you get a REAL image, larger and inverted.

When I look into the mirror it sure appears that the image is being projected from behind the mirror, so obviously one would conclude the image is VIRTUAL.

If I understand what you and JT are getting at, when we look at the mirror we are actually seeing the REAL image as predicted by optics to be in front of the mirror. How can we see this real image if there is no paper to catch it??? Are we seeing a REAL image that is floating in mid air like a hologram.

Hope you see the confusion:)

sophiecentaur
Gold Member
The definition of a real image is one that forms with the rays actually passing through it. This means that you can project it on a screen OR it appears in the space between the optical instrument and the observer.(I.e. Not behind a mirror or a lens)

jtbell
Mentor
How can we see this real image if there is no paper to catch it???
The light rays reflected off the mirror converge towards the image location, pass through it, then diverge away from it. If your eye is in the path of those diverging rays (post-image), it receives them and processes them exactly the same way as if those rays had been diverging from a real object placed at the image location.

We're so used to dealing with plane mirrors, in which the image really is virtual and located behind the mirror, that we tend to extrapolate that experience mentally to dealing with concave mirrors. It takes some experimentation and mental re-adjustment to accept that the image you see is really in front of the mirror and not behind it.

There's an amusing toy that illustrates the "reality" of a similar real image. It's basically two concave mirrors glued together around their edges, forming a "flying-saucer" shape with a mirrored interior. One mirror has a hole in the middle. You set the gadget on the table with the hole facing upward, and put a coin or other small object inside, on the lower mirror, directly underneath the hole. The mirrors are curved and arranged so they produces a real image of the object, floating right above the middle of the hole. When you look towards the hole at an angle that blocks the view of the actual object while letting the reflected rays reach your eyes, the image is so life-like that you think you can reach out and pluck it out of mid-air... but your fingers go right through it!

Are we seeing a REAL image that is floating in mid air like a hologram.
Exactly!

Doc Al
Mentor
Once again, thanks Sophie, JT, and Doc Al.

At university, taking physics courses usually means getting lost in the math and totally missing the point. Somehow I missed this huge point!

How did you guys/gals figure this out? Students and lay people would still argue that looking in a spoon, you'll see a virtual image not a floating real image.

I almost stumped my dept.head with this question today but he answered it - in a nutshell he said the same thing. It's an optical illusion, a trick played by your eye yadayada.

@jtbell: You can get a virtual and a real image from the same object at the same time.
Just imagine a object big enough. It may conjugate a part-real-part-virtual image.

@djnnt: some "paradoxes" may arise if you don't take your eyes into consideration. Remember, in order to see something, the image must be projected in your retina. So how can we see ourselves in the flat mirror? (virtual images can't be projected).

If you can read portuguese or spanish, I can send you the link of an article that explains the influence of our eyes when seeing the image. Remember: we don't see what's conjugated by the mirror (which is the image you get when doing the ray-diagram), but what's conjugated by both our eyes and the mirror. This is a point often forgotten in books and leads to some trouble when trying to apply the theory into practice.

jtbell
Mentor
Yep, that's exactly what we have, complete with the little pink pig!

How did you guys/gals figure this out? Students and lay people would still argue that looking in a spoon, you'll see a virtual image not a floating real image.
I think it first really hit me when I was playing with an optics demonstration for a general physics class 15-20 years ago. I was using an optical bench, a lamp, lens and screen, and was going to do the usual thing of locating a real image from a converging lens, using the screen. Then I was going to move the object and make a virtual image. Of course you can't capture a virtual image on a screen, but you can look through the lens, put a pencil behind the lens and above it, and then move your head around and use parallax to help you move the pencil so it points to where the image is. (I had students come up individually to try it; it was a small summer school class.)

Then I realized, "hey, I should be able to do the same thing with the real image in front of the lens!"

The same principles apply to a concave (converging) mirror, of course.

The pink piggy link helps a great deal... will be constructing a home-made version.

This is trivial but would someone like to draw a ray diagram to illustrate the concepts discussed above. Lets say for an object at 1.5 F and its rays with respect to a concave mirror.

Of course, extra rays should be included to show how the eye will see the image as seen by an eye on the objects side of the concave mirror.

Thanks!!!!!