Understanding Reflection of Light on a Concave Mirror

[jbriggs444]

As jbriggs444 said, I think this is what happening in this case

After thinking further, the fact that you see an inverted image is relevant and indicates that the person is not actually positioned between the center of curvature (C) and the focal point (F).

If the person were positioned between C and F the resulting image would be real, would be positioned on the far side of the C and would be inverted. Rays coming from the person's head would arrive at the bottom of the image. Importantly, this means that they would arrive at a downward angle. Rays coming from the person's feet would arrive at the top of the image. Importantly, this means that they would arrive at an upward angle.

But your eyes are in the way. They intercept these rays. Instead of an inverted real image forming in space to the left of the C, these rays would form an inverted real image on your retina. The rays from the person's feet would arrive at an upward angle and would illuminate the top of the retina. The rays from the person's head would arrive at a downward angle and would illuminate the bottom of the retina.

But an inverted real image on your retina is what you get when looking at an ordinary upright person. If the scenario were actually as you describe, the image you perceived and the image that your camera perceived would have both have appeared to be upright.

Accordingly, I must retract my earlier analysis and conclude that you are actually standing to the left of the C and that the real image is in front of you.

 

[hutchphd]

Replace the eye in my diagram with a camera that has a flat sensor at the back and a lens that creates an image in the plane of the sensor. Why couldn't this camera record a sharp picture?

Perhaps.
I think that (as was shown by the Hubble fix) one could likely create a thick lens that would allow a correction at a particular point along the path. But it would not be an imaging system in the normal sense of the word. The optics would image a "virtual object" I think. A negative lens in front of the camera might suffice.
Need to think a little more...maybe not today!

 

[A.T.]

I think that (as was shown by the Hubble fix) one could likely create a thick lens that would allow a correction at a particular point along the path. But it would not be an imaging system in the normal sense of the word.

It seems you would need a thinner lens than usually, because the rays are already converging. You just have to shift the convergence plane from behind the camera to the sensor.

The optics would image a "virtual object" I think.

It looks like an intermediate case: The rays used by the camera would form a real image, but they are captured, so just like with virtual images they are never focused (outside of the camera).

 

[SHASHWAT PRATAP SING]

After thinking further, the fact that you see an inverted image is relevant and indicates that the person is not actually positioned between the center of curvature (C) and the focal point (F).

If the person were positioned between C and F the resulting image would be real, would be positioned on the far side of the C and would be inverted. Rays coming from the person's head would arrive at the bottom of the image. Importantly, this means that they would arrive at a downward angle. Rays coming from the person's feet would arrive at the top of the image. Importantly, this means that they would arrive at an upward angle.

But your eyes are in the way. They intercept these rays. Instead of an inverted real image forming in space to the left of the C, these rays would form an inverted real image on your retina. The rays from the person's feet would arrive at an upward angle and would illuminate the top of the retina. The rays from the person's head would arrive at a downward angle and would illuminate the bottom of the retina.

But an inverted real image on your retina is what you get when looking at an ordinary upright person. If the scenario were actually as you describe, the image you perceived and the image that your camera perceived would have both have appeared to be upright.

Accordingly, I must retract my earlier analysis and conclude that you are actually standing to the left of the C and that the real image is in front of you.

From this, I understand that for us (our eyes or the camera) The real image of the man is formed in front of us
than what about the person(object) standing between the centre of curvature and focus.
Does this mean the person seeing in the concave mirror would see a different Real image of himself than us.
What would the person see here from his perspective...Please tell.

 

[A.T.]

What would the person see here from his perspective...Please tell.
View attachment 265811

My guess: A big blurry but not flipped image of himself.

In the video below (after 4:45) they move the camera towards the mirror. They claim the flip in the camera picture happens happens at F. But as I explained in post #35 the camera picture should flip at C, and not be flipped between C and F.

 

[SHASHWAT PRATAP SING]

My guess: A big blurry but not flipped image of himself.

Whattttttt How...?
From the rules of image formation in a concave mirror we know that-
When an object is placed between focus and the centre of curvature
A real and inverted image is formed beyond C.
then How can the person here standing between centre of curvature and focus from his perspective see a big blurry image, which is even not flipped how...?

 

[A.T.]

How can the person here standing between centre of curvature and focus from his perspective see a big blurry image, which is even not flipped how...?

See posts #35 and #71. Keep in mind that if the real image on the retina is flipped, then we see a not flipped picture. It's blurry because our eyes are trained for focusing diverging to parallel rays, not for the converging case.

 

[sophiecentaur]

How can the person here standing between centre of curvature and focus from his perspective see a big blurry image, which is even not flipped how...?

Look more carefully. The image you see will be blurry (of course) but it goes from inverted to non-inverted as you move either side of the exploding point. I really believe you are looking for something 'magic' about this rather than a rational explanation of a repeatable and identifiable situation. The only paradox is in the way you are assessing what you see.
Again, have you actually tried the spoon experiment?

 

[SHASHWAT PRATAP SING]

Look more carefully. The image you see will be blurry (of course) but it goes from inverted to non-inverted as you move either side of the exploding point. I really believe you are looking for something 'magic' about this rather than a rational explanation of a repeatable and identifiable situation. The only paradox is in the way you are assessing what you see.
Again, have you actually tried the spoon experiment?

sophiecentaur I have done that.Please read my post-#74 which is what I am confuse about...

Does this mean the real image what we are seeing and what the man is seeing is different.
would the person seeing in the concave mirror see a different Real image of himself than us ?

 

[SHASHWAT PRATAP SING]

Looking at that diagram again: Since the real image on the retina is flipped, you should see yourself not flipped in this case. Assuming your eye can still focus these rays.

So the distance at which the image on your retina flips, is not the focus point but the center of curvature. The focus point is merely the distance at which the not flipped virtual image behind the mirror is replaced by a real image behind you.

A.T. does this means that when the person is near the focus of the concave mirror he would see a big blurry but not flipped image of himself but as the person slowly moves backward when he is near the centre of curvature he would see a flipped and real image of himself in the concave mirror ?

 

[sophiecentaur]

Does this mean the real image what we are seeing and what the man is seeing is different.
would the person seeing in the concave mirror see a different Real image of himself than us ?

If we were in a different place then we would see a different image - OF COURSE. (There - now I am shouting). People see what they see, by the image on their retina and the light entering it. That image may or may not be in focus. The blurry image around the exploding zone will never be sharp for the man. If he put a correcting lens (convex) between his eye and the mirror, he could bring an image to a point in front of him and he would see it sharp but the lens would have to be way in front of him so that the image would be real, between him and the lens and beyond his 'near point'.

You say you have used a spoon but made no comment about what you saw. So I cannot comment on that but it's fairly crucial. Also, you ask @A.T. what the man 'would see'. What do you see?
If you have never done the experiment then use (borrow) a make-up /shaving mirror, if the spoon doesn't convince you.

 

[Vanadium 50]

Can we stop the bolding? If everything is bolded, nothiong is.

 

[SecularSanity]

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

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

If you say that our eyes tries to image the real image formed by mirror on the mirror itself then-

Imagine a situation where we have a concave mirror of large size and we ourself is object and we are directly seeing in the concave mirror and moving backwards so when I stand between focus and pole I see my virtual image slowly when I move backward between focus and centre of curvature then my image must be forming behind me then how my eyes are able to see my real image in the mirror….

Sir to image the real image my eyes must see the image first but in this case how without seeing the image my eyes can see the real image in the mirror it self HOW IS IT HAPPENING …..
Consider this video on youtube -https://youtu.be/7zv-4Zh-9R4

Please answer me in detail... sir i am eagerly waiting for answersView attachment 265660

A real image is seen when the light rays themselves actually converge. A virtual image is seen only where the light rays appear to converge. In other words, a virtual image is made when our eyes trace the rays back behind a mirror or lens. When you’re close to the concave mirror, between the focal point and the mirror, it’s a virtual image. The image will be upright and magnified. A real image is inverted and a virtual image is upright.

Your diagram is what’s confusing you. They’re not indicating that the image is formed behind you. They’re showing you the linear (sometimes called lateral or transverse) magnification equation.

 

[SHASHWAT PRATAP SING]

Your diagram is what’s confusing you. They’re not indicating that the image is formed behind you. They’re showing you the linear (sometimes called lateral or transverse) magnification equation.

SecularSanity please read my post-#63 and help me...

 

[SecularSanity]

SecularSanity please read my post-#63 and help me...

They’re not indicating that the object is behind. They’re showing you the magnification relative to the distance.

 

[SHASHWAT PRATAP SING]

A.T. and hutchphd please reply at my post-#80

 

[sophiecentaur]

Keep in mind that if the real image on the retina is flipped,

IMO, that's a red herring. The same thing happens with all images and all rays entering the eye. Light coming in from above the horizontal always hits the lower half of the retina.

A.T. does this means that when the person is near the focus of the concave mirror he would see a big blurry but not flipped image of himself but as the person slowly moves backward when he is near the centre of curvature he would see a flipped and real image of himself in the concave mirror ?

Why do you keep asking what someone 'would see'? If you have done the experiment (and everyone has a spoon) then the question should be 'why do I see. . . . . ..?' Do you not have access to a shaving / makeup mirror or are you, in fact a bot, coming up with the same question again and again just to wind us all up?

 

[SHASHWAT PRATAP SING]

IMO, that's a red herring. The same thing happens with all images and all rays entering the eye. Light coming in from above the horizontal always hits the lower half of the retina.Why do you keep asking what someone 'would see'? If you have done the experiment (and everyone has a spoon) then the question should be 'why do I see. . . . . ..?' Do you not have access to a shaving / makeup mirror or are you, in fact a bot, coming up with the same question again and again just to wind us all up?

I am asking the same question again and again because I am confused with this post-

The man is describing what the camera (you) are seeing not what he is seeing. He cannot see the image when it is behind his head. The camera is well beyond the center of curvature. Not a great video.

I have done the spoon experiment but in that I cannot place myself between the centre of curvature and focus of the spoon as it is at a very small distance. Due to which I am not able to understand what happens when we approach the centre of curvature and focus of the concave mirror.

The man in this video Talks as he is also seeing the same thing what we are seeing, sophiecentuar please see this video between 1:40 to 2:00


Sophiecentuar I really thank you for being engaged with me for this. ...really Thanks from my heart.

 

[sophiecentaur]

I have done the spoon experiment but in that I cannot place myself between the centre of curvature and focus of the spoon as it is at a very small distance.

I just did it. At a distance, you see yourself a long way off, upside down. You go in close and you see your eyeball the right way up. In between, there's the explosion and you see nothing because the image of your eye is behind you at first, then too near your eye to focus and then, there it is. You can tell it's the right way up when you blink.
Probably a teaspoon would be too small but I used a tablespoon and a soupspoon (spherical and better). Better still, a shaving mirror - or visit a funfair hall of mirrors.

I think you may be having a problem with confidence about understanding that video. Whatever you manage to make of it, the theory is supported by what you can actually see for yourself so you do not have to work too hard to reconcile what the guy says in the video. He is just peripheral to the real issue.
Quite frankly the video is very bad. If he wanted to show you what he was actually seeing then he should have carried the camera with him. As it is, he's actually lying to you about what he and the camera see! He's not too sharp, actually. He started off mixing up convex and concave names and then he fell over his floor marker - duh! Not worth worrying about it. Just repeat the experiment with the spoon and see your eye invert after the exploding region. Admittedly you don't see your eye in focus, up close (unless you have very strong accommodation muscles) but when you can see an image, it HAS to be in front of you.

Don't forget. You are not obliged to believe what you are shown in any video when it clearly doesn't make sense.

 

[hutchphd]

The man in this video Talks as he is also seeing the same thing what we are seeing, sophiecentuar please see this video between 1:40 to 2:00

The image he is seeing between the focal point and cc, if he has normal vision, not a clear image (it is very fuzzy and mostly indecipherable. If he is very farsighted (hyperopic) he will be able to` cheat the cc point a little.

Indeed using a negative (concave) lens one could devise a eyeball camera that could provide an in focused view for the man in the region between focal point and cc. It would not otherwise be useful, so we are better off with the cameras and eyeballs we have which work as @sophiecentaur just described and do not work well in that region.

Two things to take away: (1)an optical image means a very particular ideal one to one mapping...almost an image is not an image and (2) not all books/videos/teachers are worth your energy

If you can find a decent small mirror give it a go...nothing better than good play session. Good Luck

 

[SHASHWAT PRATAP SING]

I just did it. At a distance, you see yourself a long way off, upside down. You go in close and you see your eyeball the right way up. In between, there's the explosion and you see nothing because the image of your eye is behind you at first, then too near your eye to focus and then, there it is. You can tell it's the right way up when you blink.
Probably a teaspoon would be too small but I used a tablespoon and a soupspoon (spherical and better). Better still, a shaving mirror - or visit a funfair hall of mirrors.

I think you may be having a problem with confidence about understanding that video. Whatever you manage to make of it, the theory is supported by what you can actually see for yourself so you do not have to work too hard to reconcile what the guy says in the video. He is just peripheral to the real issue.
Quite frankly the video is very bad. If he wanted to show you what he was actually seeing then he should have carried the camera with him. As it is, he's actually lying to you about what he and the camera see! He's not too sharp, actually. He started off mixing up convex and concave names and then he fell over his floor marker - duh! Not worth worrying about it. Just repeat the experiment with the spoon and see your eye invert after the exploding region. Admittedly you don't see your eye in focus, up close (unless you have very strong accommodation muscles) but when you can see an image, it HAS to be in front of you.

Don't forget. You are not obliged to believe what you are shown in any video when it clearly doesn't make sense.

A big thankyou sophiecentaur by explaining this you have given me great satisfaction and relief. You mean a lot to me, really thankyou...

Now I clearly understand your post-(#21) ...

I understand your problem entirely. It was the same for me at first. Only when I used a mirror could I actually appreciate what's happening.

I think your problem is that you think the image is real and behind you when you can see it. When you get that condition, you are standing near the focus of the mirror and, on either side of that position, you will see an image that's either virtual and behind the mirror (when you're close) or real and in front of you (you're further away). Around that critical point, the image 'explodes' and you don't actually see it - because it's formed behind you. It's only when you go further away that the image position is actually in front of you (near and highly magnified).
Around that position your poor brain has a serious problem and can't actually make sense of what you see.
Note: This effect is most obvious with large mirrors (clearly the radius needs to be large) and the region around the exploding point is quite large. Looking in the bowl of a shiny spoon, you need to put your eye in very close and you can see your eye explode as it goes from inverted to non inverted.

sophiecentaur Please tell me now am I correct-
In real practice when we stand between the centre of curvature and focus of a large concave mirror we will see a large blurry image of ourself as our brain can't actually make sense of what we see, the actual real and inverted image is formed behind us ...

sophiecentaur Please correct me if you think...

 

[jbriggs444]

IMO, that's a red herring. The same thing happens with all images and all rays entering the eye. Light coming in from above the horizontal always hits the lower half of the retina.

It is an important point if one is under the mistaken impression that an inverted image on the retina means "seeing things upside down".

 

[SecularSanity]

I just did it. At a distance, you see yourself a long way off, upside down. You go in close and you see your eyeball the right way up. In between, there's the explosion and you see nothing because the image of your eye is behind you at first, then too near your eye to focus and then, there it is. You can tell it's the right way up when you blink.
Probably a teaspoon would be too small but I used a tablespoon and a soupspoon (spherical and better). Better still, a shaving mirror - or visit a funfair hall of mirrors.

I think you may be having a problem with confidence about understanding that video. Whatever you manage to make of it, the theory is supported by what you can actually see for yourself so you do not have to work too hard to reconcile what the guy says in the video. He is just peripheral to the real issue.
Quite frankly the video is very bad. If he wanted to show you what he was actually seeing then he should have carried the camera with him. As it is, he's actually lying to you about what he and the camera see! He's not too sharp, actually. He started off mixing up convex and concave names and then he fell over his floor marker - duh! Not worth worrying about it. Just repeat the experiment with the spoon and see your eye invert after the exploding region. Admittedly you don't see your eye in focus, up close (unless you have very strong accommodation muscles) but when you can see an image, it HAS to be in front of you.

Don't forget. You are not obliged to believe what you are shown in any video when it clearly doesn't make sense.

No, that's not correct. The image is not formed behind you at all. All of the graphs are just depicting the magnification relative to the distance.

 

[sophiecentaur]

No, that's not correct. The image is not formed behind you at all. All of the graphs are just depicting the magnification relative to the distance.

The formula
1/f = 1/u+1/v
always applies (using the correct signs, of course)
Forget magnification and just use an ordinary object / image system. Don't forget, when he's at f, the image is at infinity behind him. He even implies that when he describes what f is.
I think you are confused because you are still trying to go along with some very pretty but misleading graphics. Moving the object and the observer at the same time is a needless complication for getting to grips with the very basic system. You are trying to make two steps at once when you should really do them one at a time. That video actually does only one step because the camera doesn't move.

The fact is that there are positions in which you do not see an image as you move in and out. If you still don't believe this then that video is useless because it doesn't present the right experiment. For some reason you don't seem to want to get anything out of my spoon suggestion. In a spoon you CAN see a virtual image behind the reflecting surface and you CAN see an image in front of the surface. Not ideal but it shows the effect. In between those situations you cannot see an image. It flips behind you and you can only see it again when you are in close enough. But there's no point in going that far with the thing if you don't appreciate the 1/f = 1/u + 1/v equation.

 

[SHASHWAT PRATAP SING]

Sophiecentuar could you please reply on my post-#91 I am eagerly waiting for your reply. ...

 

[SecularSanity]

The fact is that there are positions in which you do not see an image as you move in and out. If you still don't believe this then that video is useless because it doesn't present the right experiment. For some reason you don't seem to want to get anything out of my spoon suggestion. In a spoon you CAN see a virtual image behind the reflecting surface and you CAN see an image in front of the surface. Not ideal but it shows the effect. In between those situations you cannot see an image. It flips behind you and you can only see it again when you are in close enough. But there's no point in going that far with the thing if you don't appreciate the 1/f = 1/u + 1/v equation.

I think that someone else has already pointed out that when the object is located at the focal point no image is formed because the light rays neither converge or diverge. They run parallel.
Image Characteristics for Concave Mirrors

 

[jbriggs444]

I think that someone else has already pointed out that when the object is located at the focal point no image is formed because the light rays neither converge or diverge. They run parallel.
Image Characteristics for Concave Mirrors

True, but not relevant for whether the person can see themselves in the mirror when positioned at the focal point. They can. The image that is formed is "at infinity". Images at infinity can be seen just fine.

 

[A.T.]

... when the object is located at the focal point no image is formed because the light rays neither converge or diverge. They run parallel.

True, but not relevant for whether the person can see themselves in the mirror when positioned at the focal point. They can. The image that is formed is "at infinity". Images at infinity can be seen just fine.

Much of the confusion seems to come from failing to make the distinction between "image formed by the mirror" vs "image formed by the eye/camera lens".

 

[SecularSanity]

The image isn't formed behind you.

I could be wrong, but it seems to me that if that were the case, and I was beside you, I’d be able to see your real image behind you. It’s not, it’s magnified. Look at the basic ray diagrams for a magnifying glass. When you look through it, does the object appear further away? Nope.

 

[sophiecentaur]

sophiecentaur Please tell me now am I correct-
In real practice when we stand between the centre of curvature and focus of a large concave mirror we will see a large blurry image of ourself as our brain can't actually make sense of what we see, the actual real and inverted image is formed behind us ...

Light from every part of you (over a limited range, of course) will go out and return along the same path. AS you say, that will not make any sense to your brain because the image will all be spread out, appearing to be in all directions at once.
Once you are slightly closer, there will be a normal, enlarged virtual image just like the limit where the mirror has an infinite curvature (plane). Image distance increases.

When you are more distant than the radius, I cannot help thinking it's easier to start by thinking of yourself at infinity. You are at infinity so your image will appear at f (inverted, small and definitely in front of you so you can see it). Come in a bit closer than that and your image will move out from f, towards you and get bigger. Now consider the reciprocal path; if you are at a point just outside f, you will have an image that's way behind your head.
When you are at 2f, your image will coincide with your face. (1/f = 1/2f +1/2f). Closer than 2f your image will be behind you. But, as has been pointed out and I only just really got this, your eye can still make sense of what it sees, which is a (can be highly) magnified image which is very unstable and you can only view it with one eye. Much further in and you see less and less of a proper image until you go inside the r distance and the regular shaving mirror image kicks in.
PS I just actually did this with a low quality makeup mirror and can confirm that the image is very confusing as you approach nearer to 2f. You can just see that it's inverted all the way to r then it starts to behave itself with a virtual image

 

[hutchphd]

Much of the confusion seems to come from failing to make the distinction between "image formed by the mirror" vs "image formed by the eye/camera lens".

I would take this point one step farther.
At every point along the axis (except perhaps at the focus exactly), I could design an eyeball/camera which would, in fact, project a real image of itself onto the retina (or CCD) therein. As it happens a standard camera or eyeball is not designed to do this for the contested region between between focus and cc.

 

[sophiecentaur]

The image isn't formed behind you.

I could be wrong, but it seems to me that if that were the case, and I was beside you, I’d be able to see your real image behind you. It’s not, it’s magnified. Look at the basic ray diagrams for a magnifying glass. When you look through it, does the object appear further away? Nope.

I think you must be wrong for the reason above. We are more familiar with convex lenses but the same thing happens. A close up object produces a distance image - a projector - and a distant object produces a close up image - a camera (see the formula and what I wrote above). Judging where an image actually is is not too easy. You really need to do the parallax operation as they do in school with pins on an optical bench.
This 'looking at yourself' exercise adds a lot of confusion.

 

[sophiecentaur]

I would take this point one step farther.
At every point along the axis (except perhaps at the focus exactly), I could design an eyeball/camera which would, in fact, project a real image of itself onto the retina (or CCD) therein. As it happens a standard camera or eyeball is not designed to do this for the contested region between between focus and cc.

I agree. I made the point way up the thread that a correcting lens would do the job by actually changing the focal length of the system. Problem with experimenting is that you need a very good spherical mirror to see anything much at all when closer than 2f. We all know about this stuff when applied to lenses - it's just the mirror context and the reciprocal path that makes it hard.

 

[SecularSanity]

I think you must be wrong for the reason above. We are more familiar with convex lenses but the same thing happens. A close up object produces a distance image - a projector - and a distant object produces a close up image - a camera (see the formula and what I wrote above). Judging where an image actually is is not too easy. You really need to do the parallax operation as they do in school with pins on an optical bench.
This 'looking at yourself' exercise adds a lot of confusion.

Ah, dang it! I would be able to see your real image behind you. You were right and I was wrong. I hate when that happens.

Well done!

 

[SHASHWAT PRATAP SING]

Light from every part of you (over a limited range, of course) will go out and return along the same path. AS you say, that will not make any sense to your brain because the image will all be spread out, appearing to be in all directions at once.
Once you are slightly closer, there will be a normal, enlarged virtual image just like the limit where the mirror has an infinite curvature (plane). Image distance increases.

When you are more distant than the radius, I cannot help thinking it's easier to start by thinking of yourself at infinity. You are at infinity so your image will appear at f (inverted, small and definitely in front of you so you can see it). Come in a bit closer than that and your image will move out from f, towards you and get bigger. Now consider the reciprocal path; if you are at a point just outside f, you will have an image that's way behind your head.
When you are at 2f, your image will coincide with your face. (1/f = 1/2f +1/2f). Closer than 2f your image will be behind you. But, as has been pointed out and I only just really got this, your eye can still make sense of what it sees, which is a (can be highly) magnified image which is very unstable and you can only view it with one eye. Much further in and you see less and less of a proper image until you go inside the r distance and the regular shaving mirror image kicks in.
PS I just actually did this with a low quality makeup mirror and can confirm that the image is very confusing as you approach nearer to 2f. You can just see that it's inverted all the way to r then it starts to behave itself with a virtual image

Thankyou sophiecentuar for your help and all you did for me... really thankyou

sophiecentuar I just wanted to confirm one thing by saying "Exploding Point " you mean the focus of the concave mirror, Right ?