Understanding Reflection of Light on a Concave Mirror

[A.T.]

I too have the same question but not getting an answer...

Have you read the entire thread?

Why can we see our inverted image inside a concave mirror when the image is formed in front of it the mirror and not behind?

Where are you relative to the focal point and curvature center?

 

[sophiecentaur]

Why can we see our inverted image inside a concave mirror when the image is formed in front of it the mirror and not behind?

Here's a possible reason for what you saw what you saw. It's to do with the way we 'see' images. There are many times when we see an out - of - focus image but don't ask "why?". If you are very short sighted for instance, you still identify features of the notice on the wall, even though it's not sharp. Similarly, if you are long sighted, you still recognise the layout of the book in front of you. So we don't actually need to have a focused imaged in order to 'see' it; we can identify which way up it is, though. Point is, the image can be anywhere and you will often be able to identify what you see.

If you put an object near the mirror and look at it from the appropriate distance (beyond f) you will see the familiar inverted image ( standard ray diagram). It's the position of the Object that causes the image inversion. Move your head towards the mirror and the image will still be identifiable as an inverted face) but blurred. Your brain does its best with what it sees and the inversion is the primary information about the object. By the time your face gets to where the object is, your face appears as just another blurred inverted image.

Repeat the experiment a bit off axis and the image will appear inverted and blurred but, if you put a convex (corrective`) eyeglass in the way, it will bring the light to a focus in front of your eye and you could then see a sharp image of that object.

As I have suggest higher up, you can do your experiment with a shiny spoon (it's all a bit cramped with that small radius) or with a shaving mirror (better) and confirm what I say.

 

[Vibhupriya]

Here's a possible reason for what you saw what you saw. It's to do with the way we 'see' images. There are many times when we see an out - of - focus image but don't ask "why?". If you are very short sighted for instance, you still identify features of the notice on the wall, even though it's not sharp. Similarly, if you are long sighted, you still recognise the layout of the book in front of you. So we don't actually need to have a focused imaged in order to 'see' it; we can identify which way up it is, though. Point is, the image can be anywhere and you will often be able to identify what you see.

If you put an object near the mirror and look at it from the appropriate distance (beyond f) you will see the familiar inverted image ( standard ray diagram). It's the position of the Object that causes the image inversion. Move your head towards the mirror and the image will still be identifiable as an inverted face) but blurred. Your brain does its best with what it sees and the inversion is the primary information about the object. By the time your face gets to where the object is, your face appears as just another blurred inverted image.

Repeat the experiment a bit off axis and the image will appear inverted and blurred but, if you put a convex (corrective`) eyeglass in the way, it will bring the light to a focus in front of your eye and you could then see a sharp image of that object.

As I have suggest higher up, you can do your experiment with a shiny spoon (it's all a bit cramped with that small radius) or with a shaving mirror (better) and confirm what I say.

Thank u for the answer I understood..

 

[Vibhupriya]

Have you read the entire thread?

Where are you relative to the focal point and curvature center?

What are you trying to say..

 

[A.T.]

What are you trying to say..

I'm trying to clarify your question. Are you asking about the situation as shown below? Here the visual you see is not inverted (because the image in the retina is inverted). But if you move beyond point C the visual you see will flip.

But note that this applies only if the object and eye are at the same distance from the mirror. If the camera is far away from the mirror, but the object is near the mirror, the visual flip happens when the object is at point F.

 

[sophiecentaur]

because the image in the retina is inverted

I always have a problem with that statement. Your brain has to deal with an inverted image on the retina under all conditions. Why would it be relevant here?

 

[A.T.]

I always have a problem with that statement. Your brain has to deal with an inverted image on the retina under all conditions. Why would it be relevant here?

See:
https://www.physicsforums.com/threa...ront-of-it-and-not-behind.990974/post-6364560

 

[sophiecentaur]

See:
https://www.physicsforums.com/threa...ront-of-it-and-not-behind.990974/post-6364560

More confusion here. Afaics, (your red / green diagram) if the rays from the bottom (red) of the object come from above the axis then the image is inverted. If I have an entirely different imaging system from my eye (a system involving a fan of tubes, for instance), the image will still be inverted. Can you suggest a situation when an eye and a system of tubes would disagree? That is my problem.

 

[Herman Trivilino]

I am not saying to get the converging rays directly into my eyes my main question is how my eyes can see a real image of myself which is formed behind me(As I am standing between focus and centre of curvature) in the concave mirror Please try to understand what I am saying please...

If you are facing a plane mirror and someone is standing behind you, you can see that person in the mirror. How are you able see a real person if that person is behind you?

 

[A.T.]

More confusion here. Afaics, (your red / green diagram) if the rays from the bottom (red) of the object come from above the axis then the image is inverted.If I have an entirely different imaging system from my eye (a system involving a fan of tubes, for instance), the image will still be inverted.

You have to be more specific on what you mean by "image is inverted".
a) The image in the ray diagram formed on (or near) the retina
b) The visual impression the owner of the eye has of the object

Can you suggest a situation when an eye and a system of tubes would disagree?

You have to be more specific on what you mean by "disagree". Eyes cannot disagree because they don't interpret the signals, the brains do.

 

[sophiecentaur]

Summary:: Please Explain me this problem in detail...

my image must be forming behind me then how my eyes are able to see my real image in the mirror….

You are a bit hung up on the meaning and implications of a 'real image' and this could be your problem. The terms real and virtual are used to differentiate between images for which the 'rays' diverge a point in 'real' space or when they only appear to diverge from a point somewhere.
Imagine a cinema screen with a projector forming a 'real' image on it - focused. Now move the screen forward by a short distance. There will still be an image on it. It will be real (rays are aiming at a point where they will come together) but de-focused. In your terms, you would say that the real image is behind the screen.
The same thing is going on when you have your head in front of a perfectly focused 'real ' image of yourself. To see an image, it need not be perfectly focused; many people have poor vision but can still see images. To see a sharp image in any optical system, the image (real or virtual) has to be no nearer than your limit of accommodation.

 

[sophiecentaur]

You have to be more specific on what you mean by "disagree". Eyes cannot disagree because they don't interpret the signals, the brains do.

Whether or not a brain is involved, the directions of arrival will be consistent. The eye (plus brain) or camera with a single lens will interpret the direction of arrival as being the same as a system of tubes. (This is not hard and I thought my description would be adequate.)

You have to be more specific on what you mean by "image is inverted".

I mean precisely what is meant in any other context; the visual impression. When introducing the flat lens equation, no one introduces the optics of the eye into the theory and it hardly seems necessary here. The results are always consistent because your eye works the same, whatever you look at.
When we say an image is inverted, it means that light from the top of the object arrives from below the axis, however we analyse it.

 

[A.T.]

Whether or not a brain is involved, the directions of arrival will be consistent.

If your point is that for the question "visual impression flipped or not?" we don't need to concern ourselves with the details of the eye/brain/camera, then I agree.

But the main question was "How can we (humans) see ourselves at all, if the real image formed by the mirror is behind us?". To answer this it is useful to show how the human eye forms another real image at (near) the retina. And to relate that real image to the visual impression, you have to consider the flip done by the human brain.

 

[sophiecentaur]

But the main question was "How can we (humans) can see our selves at all, if the real image formed by the mirror is behind us?". To answer this it is useful to show how the human eye forms another real image at (near) the retina. And to relate that real image to the visual impression you have to keep the flip done by the human brain in mind.

The "ourselves" issue just makes it harder. If, as I suggested, you replace the model with an object that's off axis and you look at that then the same optics is involved with a special case, when the face is the image. Put the object within f distance and you can see a sharp image at some distance greater than f (usual formula) The image will be inverted. Move your head closer and the image will be blurred (you cannot focus when it image distance is nearer than your near spot) but still inverted. Wherever you put your eye, you will still get an inverted image. Exactly the same optics applies to your eye, wherever you place it. I guess what I have been objecting to is your sudden introduction of the 'upside down' image on the retina as if it has some special significance. It's always "flipped". All that needs to be said is that the image seen gets progressively more and more blurred; nothing special happens as your eye goes past the image plane. A suitable correcting lens could bring the image to the near spot and you could see it fine.
Seeing a real image in an awkward place (without further optics to help you):
You can get precisely the same effect when standing, looking towards a projector. You can see an image without needing the screen to be there (although the visible field is limited without the screen to diffuse the incoming light. With the distances involved with a projector, it's probably easier to get the effect. You can also place your eye closer than the focal plane of a refracting telescope (without an eyepiece) and you can see the Moon or star. It's a good technique when the finder scope is in an awkward place for viewing because you have a much wider field than even a wide eyepiece. Focus is not an issue here because you just have to centre a bright blob near the centre of the tube.

 

[A.T.]

The "ourselves" issue just makes it harder.

It just means eye and object are at the same distance from the mirror. Here the visual flip occurs when they both are at twice the focal length from the mirror.

This is different from a distant camera (beyond twice the focal length). Here the visual flip occurs when the object is at one focal length from the mirror.

 

[sophiecentaur]

It just means eye and object are at the same distance from the mirror.

Exactly and it confuses the cause and effect of what is seen. The image is formed, wherever you happen to be looking from but 'so what?'.
It would seem that a real image is not an actual entity until it is observed from where it is formed - sometimes on a screen. But a projected image on a screen is actually a new Object. It's problematical to try to discuss that image if you intercepted the rays before it's been formed. We accept that a virtual image is never really there and it's just identified by our eyes but the word 'real' implies more about a real image that is strictly real.

 

[sophiecentaur]

It just means eye and object are at the same distance from the mirror.

Exactly and it confuses the cause and effect of what is seen. An image is formed, wherever you happen to be looking from.

That red / green diagram is confusing me. It seems to indicate that red and green rays all end up at points on the retina. Can that happen without another lens being introduced?
I now realize wha it is about this diagram that is misleading (or even wrong). It's what happens with the solid cones of colour entering the eye. We would agree that what we see in that position is a virtual image as it appears to be behind the mirror.

To identify (see) that image, the rays must be diverging (as if from an object in its place) as the dotted lines show. It's hard to draw a diagram which simultaneously shows both the real image (formed behind the head) and a virtual image, behind the mirror. Starting with a plane mirror, the real image is at infinity behind the observer and the virtual (mirror) image is located at same distance (behind) as the observers face. As the mirror becomes concave, the real image comes in from infinity and the virtual image recedes further behind the mirror.

As I implied earlier, there is an infinity of images, all available to be seen, wherever the observer happens to be. This is why I think the problem should be initially approached as separate object and observer.

 

[A.T.]

That red / green diagram is confusing me. It seems to indicate that red and green rays all end up at points on the retina. Can that happen without another lens being introduced?

As explained many times in this thread: A healthy/normal human eye will not form the image exactly on the retina, but near it. So the visual will be a bit blurry, but still distinct enough to tell if it's flipped. I tried it, and it works.

I now realize wha it is about this diagram that is misleading (or even wrong). It's what happens with the solid cones of colour entering the eye. We would agree that what we see in that position is a virtual image as it appears to be behind the mirror.

For an object between F and C there is no virtual image behind the mirror. Or the virtual image is at hyperinfinity.

Your diagram shows a different situation, than my diagram. The object in your diagram is not between C and F.

 

[sophiecentaur]

Your diagram shows a different situation, than my diagram. The object in your diagram is not between C and F.

Ahh. I get that. But can you show where the image is for the observer at the face position? There must be a construction which will show the rays forming a real or virtual image somewhere.

As explained many times in this thread: A healthy/normal human eye will not form the image exactly on the retina, but near it. So the visual will be a bit blurry, but still distinct enough to tell if it's flipped. I tried it, and it works.

We've both acknowledged that higher up but the red / green diagram shows it coincides with the retina. Which side is it really?

 

[A.T.]

There must be a construction which will show the rays forming a real or virtual image somewhere.

The real image formed by the mirror is behind the observer, as shown on the very left edge of my diagram.

We've both acknowledged that higher up but the red / green diagram shows it coincides with the retina. Which side is it really?

For a normal sighted person it would be in front of the retina, inside the eyeball. If the mirror has a long focal length, then a far sighted person between C and F might be able to get it on the retina and see a sharp image of the own face.

 

[sophiecentaur]

For a normal sighted person it would be in front of the retina, inside the eyeball.

So what you are saying is that the red cone has a vertex at the red o but the eye lens focuses it short of that point. Makes sense - parallel rays will focus on the retina (relaxed lens muscles). Diverging rays, further back (accommodation) and converging rays in front.

 

[A.T.]

So what you are saying is that the red cone has a vertex at the red o but the eye lens focuses it short of that point. Makes sense - parallel rays will focus on the retina (relaxed lens muscles). Diverging rays, further back (accommodation) and converging rays in front.

Before the red rays enter the eye, they are focused by the mirror on the big red O of the real image on the far left. Then the eye lens focuses them shorter on the small red o near the retina.

 

[svekola]

Kinda late, but this video may clarify some doubts. The position of “observer” is also very crucial as you will find in it.