Understanding Reflection of Light on a Concave Mirror

[A.T.]

I still don's see why you stress the optics of the eye in this.

I was just reacting to @hutchphd, who puts a focus on this. It determines how blurry the picture will be.

For the question, if the seen picture is inverted or not, it's not that important. You can simply assume a small aperture (pin-hole) for this.

 

[SHASHWAT PRATAP SING]

Sophiecentuar I just wanted to confirm that by saying r you mean the radius of curvature of the concave mirror...?

 

[hutchphd]

Thanks to @sophiecentaur and @A.T. My insistence that the entirety of the imaging lens needed a negative focal length in the "interim" region was not correct. For a fixed focus camera/eyeball only a corrective lens would need to be negative. Apologies for the oops. I think we agree.
If I can find a suitable mirror I may make a movie...I have a decent optical rail ...YOUTUBE watch out.

 

[SHASHWAT PRATAP SING]

Thank you sophiecentaur, A.T. and jbriggs444 for all you did for me...

 

[A.T.]

Thank you sophiecentaur, A.T. and jbriggs444 for all you did for me...

Thanks for your question. I learned something new myself.

 

[sophiecentaur]

Thanks for your question. I learned something new myself.

We were all looking at the subject from a new angle -literally.

 

[A.T.]

- For a distant camera the picture inversion happens when the object is at the mirror's focal point.
- For a camera at the object the picture inversion happens when the camera & object are at the mirror's center of curvature.

Just for completeness the remaining case:
- For a distant object the picture inversion happens when the camera is at the mirror's focal point.

 

[SecularSanity]

We'd have to turn around and face the real image, correct? Is there a difference between the image of the real image that we see in the mirror vs. on the screen?

Is the image that we see in the mirror just like a plane mirror image of the real image?

Is the image that we see in the mirror a true mirror image of the object?

see video

 

[A.T.]

We'd have to turn around and face the real image, correct?

Only if you put a screen where the real image is formed, and ensure appropriate lighting. Otherwise, if you turn away from the mirror you just see what is behind you.

 

[sophiecentaur]

Is there a difference between the image of the real image that we see in the mirror vs. on the screen?

Yes. Real images of that kind tend to 'hang there' and they move about as you move your head - relative to what's behind them (parallax effect) and that is unsettling for the brain. Because of the distances involved, the fact that there's 'nothing supposed to be there' and that you can also see things 'through' that real image, it doesn't look genuine. An image projected on a screen is a much more familiar thing and it behaves 'right'. These things are not indicated by a simple ray diagram. See https://www.amazon.com/dp/B089D83GXH/?tag=pfamazon01-20 that you can buy. It just looks wrong.

Is the image that we see in the mirror just like a plane mirror image of the real image?

Is the image that we see in the mirror a true mirror image of the object?

It is what it is. The point is more to do with how our brain deals with it - bearing in mind that the evolution of vision, over millions of years, didn't have to deal with high quality images in mirrors. We are still confused to some extent. Try cutting your own hair with the aid of two mirrors or drilling a tooth accurately (as a dentist does every day).

 

[A.T.]

Is the image that we see in the mirror just like a plane mirror image of the real image?

Is the image that we see in the mirror a true mirror image of the object?

An object is mirrored if an odd number of dimensions is inverted.

- Normal flat mirror inverts 1 : back&front -> you see a mirrored object

- Non-reversing-mirror inverts 2 : back&front, left&right (e.g.) -> you see a non-mirrored object

- Concave mirror with object & eye within radius of curvature: inverts 1 : back&front -> you see a mirrored object

- Concave mirror with object & eye beyond radius of curvature: inverts 3 : back&front, left&right, up&down -> you see a mirrored object

 

[SecularSanity]

An object is mirrored if an odd number of dimensions is inverted.

- Normal flat mirror inverts 1 : back&front -> you see a mirrored object

- Non-reversing-mirror inverts 2 : back&front, left&right (e.g.) -> you see a non-mirrored object

- Concave mirror with object & eye within radius of curvature: inverts 1 : back&front -> you see a mirrored object

- Concave mirror with object & eye beyond radius of curvature: inverts 3 : back&front, left&right, up&down -> you see a mirrored object

Let me see if I understand you correctly. When the image is anywhere between f and c, beyond c, or even at c, the image that we see in the mirror is only flipped vertically but it is a non-reversing image.

On the other hand, the image that we see on the screen is flipped vertically and horizontally.

Is this correct?

 

[A.T.]

Let me see if I understand you correctly. When the image is anywhere...

I didn't say anything about the location of "the image", just about the location of object & eye, and what the eye sees. Basically watching your own face in the mirror.

One reason why this thread is so long, is talking about "the image" in a scenario with multiple images.

 

[SecularSanity]

I didn't say anything about the location of "the image", just about the location of object & eye, and what the eye sees. Basically watching your own face in the mirror.

One reason why this thread is so long, is talking about "the image" in a scenario with multiple images.

Well, that’s where the confusion lies. The image formed on our eye when looking into the mirror is an inverted, non-reversing image. It’s as if we’re facing the inverted object. When we turn our head, and face the screen, as you can see, the image is an inverted mirror image of the object.

 

[sophiecentaur]

One reason why this thread is so long, is talking about "the image" in a scenario with multiple images.

Agreed.

As for the inversion business, if you are looking into mirrorworld, there will be lateral inversion always. (Clockwise goes anti-clockwise) Thereafter you can apply the rules of a lens which give no inversion when close up and (additional) complete inversion further out. (Writing will always be back to front) But looking (over your shoulder) at the distant projected image on a screen, writing will be 'legible' (clockwise goes clockwise) because you will have done another lateral inversion by looking at the 'back' of the image.

 

[SecularSanity]

Agreed.

As for the inversion business, if you are looking into mirrorworld, there will be lateral inversion always. (Clockwise goes anti-clockwise) Thereafter you can apply the rules of a lens which give no inversion when close up and (additional) complete inversion further out. (Writing will always be back to front) But looking (over your shoulder) at the distant projected image on a screen, writing will be 'legible' (clockwise goes clockwise) because you will have done another lateral inversion by looking at the 'back' of the image.

I don't think that's correct. If you take the image your eye sees in the mirror, and flip it vertically, where it's now upright, the writing would be legible. If you did the same with the image on the screen, and flipped it vertically, the image would also need to be flipped horizontally for the writing to be legible.

 

[sophiecentaur]

I don't think that's correct.

OK try this. The image in a plane mirror is laterally inverted. Reduce the radius of curvature 'a bit'. The image is still laterally inverted. That happens right in as far as 2F, after which the image flips both side to side to side and top to bottom. The lateral inversion you started with is there in addition anyway. So clockwise still goes anti-clockwise. If, as you claim, only one inversion occurred beyond 2F then which one would ti be? Would it be up down or side to side?
I think the problem is with the term 'lateral inversion' which implies side to side but it's really an inversion of handedness. Two people looking in the same mirror - one stands up and one lays down. How would you describe the difference in what the guy laying down sees, compared with the guy standing? Then rotate the sign they are reading. It doesn't help either of them to read it any easier, which is why the handedness is probably a better description of the inversion. Too late to change now.

I do a bit of astronomy and a refracting scope flips the image both ways but there is no change of handedness. (A camera at prime focus produces normal images). I have a 'star diagonal' prism which turns things ' the right way up'. That makes looking at terrestrial images much easier, for a start because up is up. But now the image is L/R flipped. That is easy to cope with but writing is wrong. This also happens with my Reflector (two mirrors in it, of course) and they produce unflipped images on a camera sensor. The implication is that the image without a secondary mirror would be laterally inverted.

 

[A.T.]

An object is mirrored if an odd number of dimensions is inverted.

- Normal flat mirror inverts 1 : back&front -> you see a mirrored object

- Non-reversing-mirror inverts 2 : back&front, left&right (e.g.) -> you see a non-mirrored object

- Concave mirror with object & eye within radius of curvature: inverts 1 : back&front -> you see a mirrored object

- Concave mirror with object & eye beyond radius of curvature: inverts 3 : back&front, left&right, up&down -> you see a mirrored object

Well, that’s where the confusion lies.

Okay, let me clarify what I mean by "mirrored" in the above post:

1) Do this with both hands:

2) Look at one hand directly, and at the other one in your mirror (or through whatever setup you have).

3) If you can rotate your hands such that both look identical (not symmetrical) to you, then your mirror/setup "mirrors" the object, in the sense I used the term. Otherwise it doesn't.

Note that stuff like text written on paper is not a good test object for this, because it is basically a 2D object. It's better to have 3 distinct axes on your object.

 

[SecularSanity]

Agreed.

As for the inversion business, if you are looking into mirrorworld, there will be lateral inversion always. (Clockwise goes anti-clockwise) Thereafter you can apply the rules of a lens which give no inversion when close up and (additional) complete inversion further out. (Writing will always be back to front) But looking (over your shoulder) at the distant projected image on a screen, writing will be 'legible' (clockwise goes clockwise) because you will have done another lateral inversion by looking at the 'back' of the image.

The reason that I think it’s incorrect is because the image that you see in the mirror when flipped vertically is non-reversing. Look at the first image in post #132. You can read the writing, but for the image on the screen, (see second image in #132) you have to turn and face it, which causes the perceived horizontal direction. When you look at the screen, it’s as if it were an inverted plane mirror image. You’re looking at the mirror image flipped vertically. It's a perfectly stamped (mirror) image.

 

[jbriggs444]

Put an imaginary frosted pane of glass at the image plane where the real image lies. Clearly it matters whether you look at the pane from the one side or the other.

 

[sophiecentaur]

you have to turn and face it,

Doing that will produce a lateral inversion. @jbriggs444 post above says it all.
Lateral inversion won't go away without another lateral inversion process.

 

[SecularSanity]

Doing that will produce a lateral inversion. @jbriggs444 post above says it all.
Lateral inversion won't go away without another lateral inversion process.

Exactly! So, that’s where all the confusion lies. The real image formed by the mirror is still a mirror image with lateral inversion.

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 it?

The answer is simple. We are able to see it because when we’re looking at the mirror, we’re seeing a mirror image of the real image itself. Therefore, the image that we see when looking into the mirror is no longer laterally inverted. Bada bing bada boom!

 

[A.T.]

The answer is simple.

Can you draw a ray diagram of what you mean?

 

[sophiecentaur]

Exactly! So, that’s where all the confusion lies. The real image formed by the mirror is still a mirror image with lateral inversion.

Because it's still in a mirror. But when there's a white screen in the way, the observer looks from the other direction and does the lateral inversion for himself. This is just as well because cameras wouldn't work otherwise. My dad had a plate camera with a ground glass screen, put in place of the plate, to do framing and focussing. That image was laterally inverted because of the viewpoint of the observer as a ten year old, I found that cool. The photo's were fine though.
Skype, Zoom etc. have the option of viewing oneself with or without mirror inversion. Seeing oneself as others see you is very confusing because we are so used to using a regular mirror. Just try combing your hair using the 'right' image.

 

[SecularSanity]

Because it's still in a mirror. But when there's a white screen in the way, the observer looks from the other direction and does the lateral inversion for himself.

Yes, exactly! And you’re right, most of us do prefer our mirror image.

Good day to you, Miss Sophie!

 

[Drakkith]

My dad had a plate camera with a ground glass screen, put in place of the plate, to do framing and focussing. That image was laterally inverted because of the viewpoint of the observer as a ten year old, I found that cool. The photo's were fine though.

I assume you looked at it from 'behind'? That would be akin to looking through a window from behind that has writing on it. The words appear to be mirrored I believe, even though there is no mirror. That's what I find interesting about mirrors. If you hold up an object and look at it in a mirror, it's almost as if the rays are going backwards from the object's surface, through the object, and into your eyes.

 

[A.T.]

I assume you looked at it from 'behind'? That would be akin to looking through a window from behind that has writing on it. The words appear to be mirrored I believe, even though there is no mirror...

That's because a plane mirror doesn't "mirror" 2D objects that are parallel to the mirror plane. A plane mirror only inverts the out-of-plane axis. And if the object has no extend or is symmetrical along that axis, then the object and mirror image are identical.

That's why 2D text is not a good test object to check the mirroring of a setup. See post #138.

 

[sophiecentaur]

Yes, exactly! And you’re right, most of us do prefer our mirror image.

Good day to you, Miss Sophie!

Oi! Young man . It’s my old boat that’s female. Not I. Read my profile before you get fruity.

 

[sophiecentaur]

I assume you looked at it from 'behind'? That would be akin to looking through a window from behind that has writing on it. The words appear to be mirrored I believe, even though there is no mirror. That's what I find interesting about mirrors. If you hold up an object and look at it in a mirror, it's almost as if the rays are going backwards from the object's surface, through the object, and into your eyes.

I seem to remember a friend's 'Russian' SLR (Zenit?) that used a mirror instead of a pentaprism to display an image the right way up. A cheap option but the image was laterally inverted. Perhaps that didn't matter with Cyrilic script - I couldn't read it even the right way round anyway.

 

[Vibhupriya]

Sir I thnk you didn't understand my question-
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 …..

THE PHOTO WAS JUST AN EXAMPLE...

SIR IN MY QUESTION JUST I MYSELF IS VIEWING ME IN THE CONCAVE MIRROR AND MOVIENG BACKWARD
SO WHEN I COME BETWEEN FOCUS AND CENTER OF CURVATURE MY REAL IMAGE WILL BE FORMED BEHIND MYSEF BUT HOW STILL I AM ABLE TO SEE MYSELF IN THE CONCAVE MIRROR HOW?
MY REAL IMAGE MUST BE FLOATING IN AIR BEHIND ME BUT STILL MY EYES ARE ABLE TO SEE IT IN THE MIRROR...HOW

I too have the same question but not getting an answer...
I was actually asking that
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?