Question regarding mirrors and optics

[Horatio]

If mirrors are flat planes of reflectance, why is it that when I make a selfie the focus scale reading for the mirrored image is essentially double that of an object on the wall beside the mirror?

Would this be due to the light/photons being reflected?

 

[Freakscene]

The optical distance is camera to surface to subject.

Marty

 

[David Hughes]

Double; it's to the mirror and back again.

Regards, David

PS But if the mirror is dirty enough the AF will focus on the dirty surface...

 

[JoeV]

Another interesting question is: Why are objects reflected in mirrors reversed horizontally but not vertically? Why just that one axis of reversal?

 

[Godfrey]

Another interesting question is: Why are objects reflected in mirrors reversed horizontally but not vertically? Why just that one axis of reversal?

It is obvious from the rule: "angle of incidence equals angle of reflection". A mirror reflects from front to back, not from left to right or top to bottom...

Here's a way to think of it, by physicist Richard Feynman (see the interview in the link below). Stand in front of a mirror, and note which direction you're facing. For the sake of this thought experiment, let's assume you're facing North. Point due East with your right hand, and your reflection points East as well. Point due west with your left hand, and your reflection gestures in the same direction. That's because these directions both lie along a plane parallel with the mirror. Similarly, point up or down and your reflection will follow suit, motioning in the same direction.

But deviate from that parallel plane even a little and thinks go wonky. Remember: your image has been reversed along the axis perpendicular to the mirror. Try pointing directly at the mirror, such that your fingertip is now directed due North. Your reflection is now pointing directly at you — not North, like your finger, but South.

Here's Dr. Feynman talking about it ... https://youtu.be/msN87y-iEx0

G

 

[retinax]

Another interesting question is: Why are objects reflected in mirrors reversed horizontally but not vertically? Why just that one axis of reversal?

Here's another way to think about it, of course it doesn't contradict the Feynman explanation:
It actually doesn't reverse left and right. Looking at our own face is the common case, but a bad example because we can't do that without a mirror. So lets say we look at a book - with script it's easy to notice left-right reversal. Stand in front of a mirror, with a book (or just imagine it). Open the book, hold it in front of your face to read. Now try to read it in the mirror. You had to turn it around to be able to see it in the mirror, right? That's what caused the reversal, not the mirror. The mirror just enables you to see the open pages when the book is facing away from you.

 

[BernardL]

Another interesting question is: Why are objects reflected in mirrors reversed horizontally but not vertically? Why just that one axis of reversal?

Godfrey argues that the image is not reversed; yet it is reversed in some way, because that book is so hard to read in the mirror image. The crux of the quoted question is boldfaced. I believe the explanation is: if you imagine to move your body and put yourself alongside your double in virtual space, you will do it standing upright, for your comfort. Then "up" is the same for both, but once you stand besides your double, your right is his left.

But... you could have made that visit by doing a half-somersault, standing on your hands next to your double, facing the same direction. Then your right is same as his right, but your top is his bottom.

Or, wrapping up, the answer to the boldface above is: depends on your acquired habits for moving around.

 

[retinax]

Yes. You can also turn the book upside down rather than left-right to view in the mirror, and left-right will be unchanged. Indeed that we observe left-right, but not top-bottom reversal in the mirror, is because we typically turn around left-right to face it, rather than top-bottom.

 

[Horatio]

Google helped me find this explanation (Italics mine), although it does not answer why:

Characteristics of Plane Mirror Images
Objects placed in front of plane mirrors will have a corresponding image located behind the mirror. The distance from the image to the mirror is always identical to the distance from the object to the mirror. So if a person stands 2.0 meters in front of the mirror, then the image will be located an identical 2.0 meters behind the mirror. Such an image is a virtual image; when viewing such a virtual image in the mirror, it would seem as though light is coming from a location 2.0 meters behind the mirror. If you were to walk behind the mirror and look at this so-called virtual image location, there would be nothing physical present there. It only seems to the observer as thought light is coming from this location to the eye when viewing the image of the person in the mirror.

A further link:

Virtual vs. Real Images
In the case of plane mirrors, the image is said to be a virtual image. Virtual images are images that are formed in locations where light does not actually reach. Light does not actually pass through the location on the other side of the mirror; it only appears to an observer as though the light is coming from this location. Whenever a mirror (whether a plane mirror or otherwise) creates an image that is virtual, it will be located behind the mirror where light does not really come from. Later in this unit, we will study instances in which real images are formed by curved mirrors. Such images are formed on the same side of the mirror as the object and light passes through the actual image location.

I take this to mean if the image were real, the distance would not double.

https://www.physicsclassroom.com/class/refln/u13l2b.cfm

 

[Godfrey]

Godfrey argues that the image is not reversed; yet it is reversed in some way, because that book is so hard to read in the mirror image. ...

Wrong: I did NOT say that the image is not reversed.

I said that it is reversed in a way that is different from left to right or top to bottom: it is reversed in the direction of front to back, orthogonal to the plane of the mirror, as Dr. Feynman's thought experiment demonstrates. This produces the effect in our perception of being reversed left to right, or top to bottom, depending upon our point of view (or orientation) with respect to the plane of the mirror.

G

 

[JoeV]

Well, I didn’t mean to start an argument! I think Martin Gardener addressed this issue in his book The Ambidextrous Universe, but I’ve long misplaced my copy.

At one time I thought the apparent left/right reversal was because of binocular vision, but obviously it happens with one eye closed too.

Hopefully I won’t be asking any of you how to focus at infinity. ��