The only thing planar mirrors reverse is front-back

The only thing planar mirrors reverse is front-back.
http://upload.wikimedia.org/wikipedi...versal.svg.png

On Tuesday, 24 February 2015 11:39:13 UTC-5, Sam Wormley wrote:
The only thing planar mirrors reverse is front-back.
http://upload.wikimedia.org/wikipedi...versal.svg.png

Cute diagram. A planar surface acting like a convex lens...

On 2/24/15 4:57 PM, RichA wrote:
On Tuesday, 24 February 2015 11:39:13 UTC-5, Sam Wormley wrote:
The only thing planar mirrors reverse is front-back.
http://upload.wikimedia.org/wikipedi...versal.svg.png

Cute diagram. A planar surface acting like a convex lens...


Um no -- How about a 45 degree diagonal for your telescope.

On Tuesday, February 24, 2015 at 9:39:13 AM UTC-7, Sam Wormley wrote:
The only thing planar mirrors reverse is front-back.

http://upload.wikimedia.org/wikipedi...versal.svg.png

That's true, but I don't see how the image you linked to makes that clear.

Yes, it shows that front and back _are_ reversed, but one would need something
different to explain the left-right illusion.

Let us say one has a mirror encompassing the north wall of a room which, like
most rooms we know, has the shape of a rectangular prism.

You turn and face the mirror.

You see your image, and it certainly looks like you with the right and left
sides reversed.

But symmetry considerations don't allow the mirror to tell the difference
between the up-down direction and the sideways direction. So what's going on?

Up and down aren't reversed.

And *east and west* aren't reversed. You face north, and stretch out your right
hand - it points east. So does the corresponding hand on your mirror image.

Front and back are reversed - your mirror image faces south, you face north.
But that is a change in orientation, isn't it, not a change in shape, like a
parity reversal?

But front and back weren't reversed by rotation - east and west stayed put. So
the front and back reversal is the cause of the parity reversal. Which is
described as reversing right and left because of how right and left are
*defined*. Looking down from the top, rotate 90 degrees from "front", and you
get "right".

With front and back reversed, as in the mirror image, the right side of the
mirror image is on the west while your right side is on the east.

East and west are not reversed, any more than top and bottom. But left and
right are reversed *because their definition involves using front and back*.

John Savard

On Wednesday, 25 February 2015 00:04:13 UTC+1, Sam Wormley wrote:

Um no -- How about a 45 degree diagonal for your telescope.

Ah, but a diagonal on a refractor produces a beneficial inversion to "right way up" but the image is laterally reversed.

The poor man's terrestrial telescope. :ø))

On 2/24/2015 7:09 PM, Quadibloc wrote:
On Tuesday, February 24, 2015 at 9:39:13 AM UTC-7, Sam Wormley wrote:
The only thing planar mirrors reverse is front-back.

http://upload.wikimedia.org/wikipedi...versal.svg.png

That's true, but I don't see how the image you linked to makes that clear.

Yes, it shows that front and back _are_ reversed, but one would need something
different to explain the left-right illusion.

Let us say one has a mirror encompassing the north wall of a room which, like
most rooms we know, has the shape of a rectangular prism.

You turn and face the mirror.

You see your image, and it certainly looks like you with the right and left
sides reversed.

But symmetry considerations don't allow the mirror to tell the difference
between the up-down direction and the sideways direction. So what's going on?

Up and down aren't reversed.

And *east and west* aren't reversed. You face north, and stretch out your right
hand - it points east. So does the corresponding hand on your mirror image.

Front and back are reversed - your mirror image faces south, you face north.
But that is a change in orientation, isn't it, not a change in shape, like a
parity reversal?

But front and back weren't reversed by rotation - east and west stayed put. So
the front and back reversal is the cause of the parity reversal. Which is
described as reversing right and left because of how right and left are
*defined*. Looking down from the top, rotate 90 degrees from "front", and you
get "right".

With front and back reversed, as in the mirror image, the right side of the
mirror image is on the west while your right side is on the east.

East and west are not reversed, any more than top and bottom. But left and
right are reversed *because their definition involves using front and back*.

John Savard

Good lord the ignorance



"Sometimes, however, we come across a completely different "explanation"
of "what mirrors do", the idea being that mirrors actually reverse
things "front-to-back""

"The most interesting thing about this "explanation" is that it's
clearly based on a counter-factual premise. The "chair" shown on the
right side of this drawing does not exist. What really exists are
reflected rays of light bearing a particular relationship to the direct
rays of light from the chair. The corresponding images, which are 2D
projections of those families of light rays for a particular observer,
appear reversed about the axis of the observer's rotation."

Do try and learn a bit before you make counter-factual claims


http://www.mathpages.com/home/kmath441/kmath441.htm

David Staup wrote:
On 2/24/2015 7:09 PM, Quadibloc wrote:
On Tuesday, February 24, 2015 at 9:39:13 AM UTC-7, Sam Wormley wrote:
The only thing planar mirrors reverse is front-back.

http://upload.wikimedia.org/wikipedi...versal.svg.png

That's true, but I don't see how the image you linked to makes that clear.

Yes, it shows that front and back _are_ reversed, but one would need something
different to explain the left-right illusion.

Let us say one has a mirror encompassing the north wall of a room which, like
most rooms we know, has the shape of a rectangular prism.

You turn and face the mirror.

You see your image, and it certainly looks like you with the right and left
sides reversed.

But symmetry considerations don't allow the mirror to tell the difference
between the up-down direction and the sideways direction. So what's going on?

Up and down aren't reversed.

And *east and west* aren't reversed. You face north, and stretch out your right
hand - it points east. So does the corresponding hand on your mirror image.

Front and back are reversed - your mirror image faces south, you face north.
But that is a change in orientation, isn't it, not a change in shape, like a
parity reversal?

But front and back weren't reversed by rotation - east and west stayed put. So
the front and back reversal is the cause of the parity reversal. Which is
described as reversing right and left because of how right and left are
*defined*. Looking down from the top, rotate 90 degrees from "front", and you
get "right".

With front and back reversed, as in the mirror image, the right side of the
mirror image is on the west while your right side is on the east.

East and west are not reversed, any more than top and bottom. But left and
right are reversed *because their definition involves using front and back*.

John Savard

Good lord the ignorance



"Sometimes, however, we come across a completely different "explanation"
of "what mirrors do", the idea being that mirrors actually reverse things "front-to-back""

"The most interesting thing about this "explanation" is that it's clearly
based on a counter-factual premise. The "chair" shown on the right side
of this drawing does not exist. What really exists are reflected rays of
light bearing a particular relationship to the direct rays of light from
the chair. The corresponding images, which are 2D projections of those
families of light rays for a particular observer, appear reversed about
the axis of the observer's rotation."

Do try and learn a bit before you make counter-factual claims


http://www.mathpages.com/home/kmath441/kmath441.htm

Page after page of waffle. Might as well be discussing how many angels can
dance on the head of a pin.
In the mirror left reflects left, right reflects right, top reflects top
and bottom reflects bottom, The image seems to be left/right inverted
because it's inverted front /back.

On Tuesday, February 24, 2015 at 6:04:13 PM UTC-5, Sam Wormley wrote:
On 2/24/15 4:57 PM, RichA wrote:
On Tuesday, 24 February 2015 11:39:13 UTC-5, Sam Wormley wrote:
The only thing planar mirrors reverse is front-back.
http://upload.wikimedia.org/wikipedi...versal.svg.png

Cute diagram. A planar surface acting like a convex lens...


Um no -- How about a 45 degree diagonal for your telescope.

The explains the reversal, not the convergence. No lens = no convergence.

On Tuesday, February 24, 2015 at 11:39:13 AM UTC-5, Sam Wormley wrote:
The only thing planar mirrors reverse is front-back.
http://upload.wikimedia.org/wikipedi...versal.svg.png

We are not seeing the "back" of the arrow, we are still seeing the "front."

wrote:
On Tuesday, February 24, 2015 at 11:39:13 AM UTC-5, Sam Wormley wrote:
The only thing planar mirrors reverse is front-back.
http://upload.wikimedia.org/wikipedi...versal.svg.png

We are not seeing the "back" of the arrow, we are still seeing the "front."

Exactly. If the front and back were not reversed you would see the back in
the mirror which is obviously not possible.