BS: Reflection & the behaviour of light

Subject: BS: Reflection & the behaviour of light
From: MGM·Lion
Date: 02 Feb 11 - 06:01 AM

Since I learnt, in Physics in the 3rd year at grammar school in 1945-46, about the behaviour of light waves, and why this explains the left-right reversal of a mirror-image, I have been exercised by the question as to why this side-to-side reversal does not also occur as a bottom-to-top reversal. The Physics teacher, one Mr Walters, was unable to explain this phenomenon to my comprehension; and many of my classmates agreed that they shared my puzzlement as to the reason.

Can anyone on Mudcat please explain, in terms sufficiently simple, not over-technical and non-mathematical, to be comprehensible to a scientific ninny like me, why this absence of up-down reversal when there is side-side reversal? Please!

~Michael~

Subject: RE: BS: Reflection & the behaviour of light
From: gnu
Date: 02 Feb 11 - 06:58 AM

There is none.

Subject: RE: BS: Reflection & the behaviour of light
From: Will Fly
Date: 02 Feb 11 - 07:00 AM

With regard to a reversed image, you need to answer the question: "Reversed relative to what?" Imagine you're looking at a rear-view mirror in a car. The "reversal" is a reflection and refers to the image in the rear-view mirror being reversed relative to the image you would see if you turned around and looked directly out the back window. If this is so, then the answer depends on HOW you would turn around. Most likely you would rotate our field of view about a vertical axis, so the image you see will be reversed laterally (left-to-right). Of course, if you turn about a horizontal axis, the image you see would be reversed top-to-bottom.

This is an answer I was given many years ago by a physics teacher. You might as well ask why all points of an image aren't reflected at all points - a bit like a kaleidoscope...

Subject: RE: BS: Reflection & the behaviour of light
From: Will Fly
Date: 02 Feb 11 - 07:04 AM

I should have added that the physics of a plane mirror are quite complex. If you go through a Hall of Mirrors in a funfare, you'll see the distortion that occurs because of the curvature of a mirror. If you look at your face in a spoon, you'll get a vertical and horizontal reflection on one side and a horizontal, distorted reflection on the other.

Subject: RE: BS: Reflection & the behaviour of light
From: Black belt caterpillar wrestler
Date: 02 Feb 11 - 07:45 AM

The thing about the behaviour of light that puzzles me is how does it travel through clear solids like glass and not through others like steel. The first thought is that it is something to do with crytal structure but I thought that glass was not a crystal but an amorphous super-cooled liquid. The second thought is that perspex is not a supercooled-liquid.
Confused!

Subject: RE: BS: Reflection & the behaviour of light
From: DMcG
Date: 02 Feb 11 - 08:05 AM

There isn't really a left to right reversal at all, physically speaking - there is only a front-to-back. Then, as Will Fly explained, if you try to superimpose yourself on that reversal, the easiest way to do so is to call it a left to right reversal.

Here's a simple, if slightly insane, experiment. Put a brown paper bag on the hand of your choice, and wear a hat with a large feather sticking out the back of it (preferably one an emblem on the front). Now, stand in front of the mirror and move the bagged hand. The image moves its bagged hand. Same with the unbagged hand. There is no reversal. All that has happened is that you *want* to call the bagged hane 'left' on you and 'right' on the image. With the hat, feather and emblem you can confirm what has really changed is the front-to-back.

You can explain all of this to the nice men in the white coats ...

Subject: RE: BS: Reflection & the behaviour of light
From: Bobert
Date: 02 Feb 11 - 08:13 AM

Lay on yer side and look at the mirror...

Subject: RE: BS: Reflection & the behaviour of light
From: GUEST, topsie
Date: 02 Feb 11 - 08:22 AM

Look at your reflection in a shiny spoon.

Subject: RE: BS: Reflection & the behaviour of light
From: DMcG
Date: 02 Feb 11 - 08:28 AM

Look at your reflection in a shiny spoon
Actually, that's a really interesting one. You will immedately spot the top & bottom are reversed. It takes a mosment longer to discover that (apparently) left and right are not.

However, moving to thinking about mirrors that aren't flat makes the whole subject even more confusing for most people.

Subject: RE: BS: Reflection & the behaviour of light
From: GUEST,Patsy
Date: 02 Feb 11 - 08:29 AM

The reflection in the spoon is the best example - when I was little my father told me there was a little upside down girl in the spoon.

Subject: RE: BS: Reflection & the behaviour of light
From: MGM·Lion
Date: 02 Feb 11 - 08:37 AM

Thanks for responses so far ~~ but I fear I remain none the wiser or more enlightened:

"Left Right; Left Right --

and all together now ~~

♫ Oh, I live alongside of the Ups-and-Downs
Fol-de-rol-diddlum-day...♪♪



~leahciM~

Subject: RE: BS: Reflection & the behaviour of light
From: Dave MacKenzie
Date: 02 Feb 11 - 08:52 AM

Maybe Rene Magritte knew more than we think.

Subject: RE: BS: Reflection & the behaviour of light
From: Bill D
Date: 02 Feb 11 - 10:32 AM

All points in a perfectly flat mirror will be reflected directly back in relation to their original position.

The 'left-right' reversal is only in the mind. The mirror doesn't 'do' anything except show a.....well, a mirror image. The 'left eye' think you see in the mirror is actually the right eye. Imagine a photographic negative that can be viewed from either side. Make a dot on the left eye, then look at it from the back. The dot hasn't moved just because you now see it 'on the other side'...and turning the negative upside down doesn't change the relative positions of anything.

All this is why, in telescopes and binoculars and cameras, extra lenses are used to reverse images a 2nd time so that we DO see left & right the way we would 'normally'.

Subject: RE: BS: Reflection & the behaviour of light
From: Van
Date: 02 Feb 11 - 12:11 PM

Is it not a mental matter as much as a question of Physics. Think about it. When you stand facing someone their left hand "faces" your right hand and vice versa this how your brain expects to see the image. Just a thought. There's probably some bloody great mathematical formula that sane man can understand but logically it should reverse the blooming lot.

Subject: RE: BS: Reflection & the behaviour of light
From: MGM·Lion
Date: 02 Feb 11 - 01:15 PM

Exactly, Van: it should ~~ including in vertical as well as horizontal plain. Which is where I came in...

Subject: RE: BS: Reflection & the behaviour of light
From: Newport Boy
Date: 02 Feb 11 - 01:26 PM

DMcG has the explanation right. It is a combination of language and interpretation by the brain. The image of your left side is on the left in the mirror, so where's the reversal? If you imagine yourself standing behind the image and looking out, having turned through 180 degrees, the bag on your (real) left hand would be on the opposite side to the bag on the (image) left hand. It's your brain that's created the 'reversal' when you look at the mirror from the front.

This mental reversal is extremely powerful. BillD mentions the additional inverting lenses in telescopes and binoculars. These were not used in surveying instruments (theodolites and levels), and you soon get used to working with an inverted, reversed image.

Phil

Subject: RE: BS: Reflection & the behaviour of light
From: MGM·Lion
Date: 02 Feb 11 - 01:32 PM

No, Phil. If I stand facing you, your right hand is on my left. If I say "Let's both hold up our right hands", we will hold up hands diagonal to one another. But if I stand facing a plain flat mirror and hold up my right hand, the mirror-image holds up what would be your left hand if I were facing you, relative to my position. Do you not see that?

~M~

Subject: RE: BS: Reflection & the behaviour of light
From: Bill D
Date: 02 Feb 11 - 02:36 PM

A LENS would reverse things top to bottom...not a mirror. In a full length mirror, you can see your feet also 'seeming' to be reversed left for right, but try to imagine how they could possibly be at the top. It's just that the IDEA of your mirror image being backwards is so strong. If you have two mirrors, you can counter the 'reversal' in the 2nd one, but you still don't get an up-down change.

Subject: RE: BS: Reflection & the behaviour of light
From: Bill D
Date: 02 Feb 11 - 02:38 PM

Think of it this way... you are not 'reversing' your left & right, but simply 'reflecting' your left & right.

Subject: RE: BS: Reflection & the behaviour of light
From: MGM·Lion
Date: 02 Feb 11 - 02:42 PM

No, Bill, I think not. The concept of 'idea' is not cognate with the concept of 'perception', which you seem to me to be implying. Or, to put it differently, you are begging the question [in the correct philosophical sense of ignoratio elenchi] as to where such an 'idea' would, or could, have come from in the first place.

~M~

Subject: RE: BS: Reflection & the behaviour of light
From: The Fooles Troupe
Date: 02 Feb 11 - 02:51 PM

"additional inverting lenses in telescopes and binoculars. These were not used in surveying instruments (theodolites and levels), and you soon get used to working with an inverted, reversed image."

Unless things have changed, astronomical telescopes see the image inverted and mirrored.

Subject: RE: BS: Reflection & the behaviour of light
From: Bill D
Date: 02 Feb 11 - 02:53 PM

But Michael... the mirror has neither 'idea' nor 'perception'. This conundrum has been linguistically juggled for decades, if not centuries, and the only way to deal with it is to realize that we, with our concepts and language are asking questions that physics has no problem with.

Ask yourself how you'd design a 'mirror' which WOULD give both types of reversal. What you are doing is assigning the name of 'right arm' to the one in the reflected version, as if it were another entity. The 'right arm' of the mirror entity IS your left arm. Nothing is backwards.

Subject: RE: BS: Reflection & the behaviour of light
From: DMcG
Date: 02 Feb 11 - 02:59 PM

It takes a bit of doing, but try this mental task. Imagine a right hand glove with the thumb towards you. Then imagine every point moving so that it gets flatter and flatter until it is completely flat. Then - impossible in the real world - let them keep moving until the thumb is as far behind the fingers than it was in front. You will find that this front-to-back change has also changed it from a right-hand to a left-hand glove

Subject: RE: BS: Reflection & the behaviour of light
From: Paul Burke
Date: 02 Feb 11 - 03:15 PM

Look at the image and compare it with your surroundings:

What's at the top relative to you, is at the top of the image relative to you.
What's at the bottom relative to you, is at the bottom of the image relative to you.
What's on the left relative to you, is on the left of the image relative to you.
What's on the right relative to you, is on the right of the image relative to you.

No reversal. No conundrum. No idea worth talking about.

Subject: RE: BS: Reflection & the behaviour of light
From: gnu
Date: 02 Feb 11 - 03:39 PM

That's what I said... simply.

Subject: RE: BS: Reflection & the behaviour of light
From: MGM·Lion
Date: 02 Feb 11 - 03:40 PM

Yes, Paul ~~ but on the opposite side of the image-figure of me: think about that ~~ what is on my right is on the left of the mirror-image of me, and appears reversed into the bargain.

Subject: RE: BS: Reflection & the behaviour of light
From: MGM·Lion
Date: 02 Feb 11 - 03:42 PM

... One such object being a picture on the wall behind me, whose reversal can therefore be particularly easily identified.

Subject: RE: BS: Reflection & the behaviour of light
From: Newport Boy
Date: 02 Feb 11 - 04:34 PM

The problem is in the use of language - there is no 'reversal' - it's just a reflection. How about this logical argument:

1. Looking in the mirror with a bag on your right hand, you see an image with a bag on "its left hand". (The image doesn't have a 'left' hand - it's only an image - left and right are your perception.)

2. Have an 'out-of-body' experience - let your virtual self walk around and stand behind the image. Your virtual image has a bag on the hand on the left side of the mirror - "its right hand". The mirror image has a bag on the right side of the mirror - "its left hand".

3. Your virtual self has turned 180 degrees to get to this position, and has therefore undergone a left-right reversal. Therefore, since the bag on the mirror image is on the other hand, it has not undergone a left-right reversal.

Phil

Subject: RE: BS: Reflection & the behaviour of light
From: Bill D
Date: 02 Feb 11 - 05:17 PM

This is conceptually similar to Zeno's Paradox, in which he 'shows' that we can never move from A to B, when we know that we DO move. We merely have linguistic ambiguities that allow us to express ideas that seem impossible.

Subject: RE: BS: Reflection & the behaviour of light
From: s&r
Date: 02 Feb 11 - 06:59 PM

The man in the mirror hasn't turned round: his left is your left you're just looking at his front image. If you removed your skin and stuck it on the mirror (ignoring the impossibility) you would see yourself just as much 'reversed'.

Stu

Subject: RE: BS: Reflection & the behaviour of light
From: Joe_F
Date: 02 Feb 11 - 07:43 PM

Another example: Suppose you are holding a book vertically in front of you. You want to see how it looks in the mirror, so you flip it to make it face the mirror. If *you* flip it left to right, the mirror image will be reversed left to right. If *you* flip it top to bottom, the mirror image will be reversed top to bottom.

Suppose, instead you have a text printed on a transparent sheet, so you can view it in the mirror without rotating it. Then there is no reversal of the text (except front to back, which doesn't matter).

We say that our own images are reversed from left to right because that corresponds to the (approximate) symmetry of the human body. It is less odd to say "The guy in the mirror is left-handed" than to say "The guy in the mirror has his face on the back of his head".

Subject: RE: BS: Reflection & the behaviour of light
From: olddude
Date: 02 Feb 11 - 07:44 PM

What is fun is take the laser out of a blue ray DVD writer, then replace that diode in a standard laser pointer. you can burn holes in stuff with it ... cool

Subject: RE: BS: Reflection & the behaviour of light
From: Bill D
Date: 02 Feb 11 - 07:57 PM

cool? You mean HOT?

And you learned this how? *grin*

Subject: RE: BS: Reflection & the behaviour of light
From: Slag
Date: 02 Feb 11 - 08:52 PM

Reflection is termed a "1 to 1 correspondence" which is to say that reflected light is like an exact representation of what is before the reflector. We assign "handedness" as a convention and mostly because we have bilateral symmetry. Consider holding a starfish before the mirror or better yet a ball. There is no "handedness" to lend confusion to the issue. You see the placement of incidental features in their corresponding positions, point for point.

Here's another example. Find a rubber stamp with an image or writing depicted on it. It looks reversed to you. Ink it and now take it to the mirror. Slowly approach the mirror. The image "looks" correct in the mirror regardless of up or down orientation. As you press it on to the mirror every point of contact is correctly correspondent to the object, the stamp itself and as you pull it away, the inked print is also "looks" correct to your eyes. Now quick with the alcohol and clean it off before the wife sees it!

If you want to reduce the question to quanta, just think of the photons as little billiard balls bouncing back at you. I know they are really being absorbed and re-emitted in most cases but the idea is the same. The light from your body and clothing came from an original emitter at many different wave-lengths. Some are absorbed by you and your clothing and bodily features and that which could not be absorbed reflects off you in many directions. Those going in the direction of the mirror encounter a very high degree of reflectivity. That is, almost all the light bounces off without a change in energy levels (wave-length). Some of that reflcted light hits our pupils and the rest you know (or should know).

Yes OD! Check out Sam's Laser FAQ!

Subject: RE: BS: Reflection & the behaviour of light
From: Donuel
Date: 02 Feb 11 - 09:37 PM

Light that is visible to the human eye is a tiny slice of the light (electro magnetic waves) which we know exists. Some of the other frequencies of light can "shine" through steel. Gamma waves can shine through damn bear anything. Then of course there are the neutrinos which can shine through planets or even stars.

An interesting fact is that you can make light move at different speeds depending upon the material you shine the light through.
It slows down in glass and shows its changes in refracted light. It slows even more in diamond and refracts at even greter angles.
In super cooled fields of gas, light has been made to virtually stop.

Subject: RE: BS: Reflection & the behaviour of light
From: Amos
Date: 02 Feb 11 - 10:51 PM

MtheGM:

Every point on your body's front is directly reflected in a flat mirror straight across from its physical location. Imagine the points of the body connected to their reflected corresponding points in the mirror. The ones on your left shoulder would go straight over to the side of the mirror on your left. The ones on your right go straight over to the side of your mirror that is to your right. This is the simplest possible scheme of straight-line reflection.

For your mirror image to be "correct" from your point of view of it's point of view all those straight-line reflections would have to cleverly find their way to the opposite side. Very complex, improbable behavior for one ray let alone millions of them.

Just trace the rays and it will make sense.

Subject: RE: BS: Reflection & the behaviour of light
From: MGM·Lion
Date: 02 Feb 11 - 11:47 PM

Ah ~~ the lengths of some of the responses, the knots you are all tying yourselves into, the peculiar suggestions you are making ~~ all seem to me to re-establish the points made in my OP, and the difficulties experienced by poor old Wally Walters back when I was 13, at that.

Many thanks for your efforts all.

~Michael~

Subject: RE: BS: Reflection & the behaviour of light
From: Newport Boy
Date: 03 Feb 11 - 07:18 AM

It all reminds me of the old tale. In my younger days, motorcyclists wore long leather coats with buttons (I later wore my father's). On a winter's day, one poor chap got very cold with the wind getting in between the buttons. He hit on a solution - he turned the coat around and buttoned it up the back.

He must have ridden faster after that, and unfortunately crashed. He was dead when the police arrived - they asked the people first on the scene "Was he dead when you found him?"

"Well, I'm not sure - he was badly injured, but he seemed to be conscious until we turned his head the right way round".

(It all depends on your perception.)

Phil

Subject: RE: BS: Reflection & the behaviour of light
From: DMcG
Date: 03 Feb 11 - 07:35 AM

Ah ~~ the lengths of some of the responses, the knots you are all tying yourselves into, the peculiar suggestions you are making ~~ all seem to me to re-establish the points made in my OP

I think the length is simply due to trying to explain in detail, and the examples are just to assist.

There are two short answers, both correct:

(i) There is no (physical) problem - as reported by gnu, Paul Burke, myself and others

(ii) There is a linguistic problem because the concept of 'left' is more subtle than it seems - as reported by Bill D, Newport Boy, myself and others. But this is really a problem about words and the ideas they describe, so there's no reason a mirror even needs to be involved. If you want to go further on this aspect, expect much more complex and surprising results than looking at a mirror will give you. A few years studying Wittgenstein, for example, is probably a good start.

Subject: RE: BS: Reflection & the behaviour of light
From: Bill D
Date: 03 Feb 11 - 11:53 AM

"A few years studying Wittgenstein,..."

from memory:
"Was heisst es war ein spiel ist?" "What does it mean to know what a game is?"

Now THAT will keep you going longer than mumbling over mirrors.

Subject: RE: BS: Reflection & the behaviour of light
From: JohnInKansas
Date: 03 Feb 11 - 12:40 PM

I'm glad he didn't ask:

Why does the image you see when you look through a spinning Dove prism rotate twice as fast as the prism does?

or:

When the "TV camera" zooms in on the players at a ball game, why do the players farther from the camera look taller than the ones closer to it?

Or one of those other simple optical conundrums.

John

Subject: RE: BS: Reflection & the behaviour of light
From: GUEST, topsie
Date: 03 Feb 11 - 03:01 PM

illusions like the wheels going backwards in films of stagecoaches?

Subject: RE: BS: Reflection & the behaviour of light
From: Dave MacKenzie
Date: 03 Feb 11 - 03:22 PM

"why do the players farther from the camera look taller than the ones closer to it?"

Because the zoom lens distorts perspective, or at least misleads the eye's points of reference.

Reminds me of Father Ted: "This cow is small, Dougall, and those are far away"

Subject: RE: BS: Reflection & the behaviour of light
From: Bill D
Date: 03 Feb 11 - 04:00 PM

Lucy to Charlie Brown:"Did you ever notice how way off in the distance, the sidewalk comes to a point?"

Charlie Brown, in exasperation: "Lucy, the sidewalk doesn't come to a point. That's just an illusion. It's called 'perspective'."

Lucy stares at him....then "Charlie Brown, why do you always wear that shirt with the stupid stripe on it?"

Subject: RE: BS: Reflection & the behaviour of light
From: MGM·Lion
Date: 03 Feb 11 - 06:06 PM

Oh, good grief!

Subject: RE: BS: Reflection & the behaviour of light
From: gnu
Date: 03 Feb 11 - 06:29 PM

Lucy is a tease. If I was Chuck I would kicked more than the football and shed some light as well as reflection on that bitch.

Subject: RE: BS: Reflection & the behaviour of light
From: MGM·Lion
Date: 04 Feb 11 - 03:19 AM

But, gnu, surely the whole point is that poor old Charlie Brown never does actually get to kick that football.

And it is only the poor, pathetic, homely & unloved Peppermint Patty who ever calls him "Chuck".

~M~

Nuff drift now ~~ back to the vital poits at issue, everybody, please. Come now: jump to it!

~The OP!~

Subject: RE: BS: Reflection & the behaviour of light
From: MGM·Lion
Date: 04 Feb 11 - 03:46 AM

That is, poiNts.

Oh for a post-posting editing facility, as someone else pleaded recently.

Subject: RE: BS: Reflection & the behaviour of light
From: Slag
Date: 04 Feb 11 - 09:48 AM

Question aked was answered, accurately with all necessary and sufficient information. If it is word games and semantics you wanted, you should have said so!

Subject: RE: BS: Reflection & the behaviour of light
From: Amos
Date: 04 Feb 11 - 10:52 AM

No need getting all wrapped around an axle, MgM--there's nothing strange going on. What you see is a direct point-for-point reflection of what you are sending out (in light).

PErhaps you need to define the problem again. Do you seriously think, given what you know of light and space, that the inage in a mirror should come back to you flipped across just to match your idea of handedness?


A

Subject: RE: BS: Reflection & the behaviour of light
From: freda underhill
Date: 04 Feb 11 - 12:23 PM

It's all relative

Subject: RE: BS: Reflection & the behaviour of light
From: Geoff the Duck
Date: 04 Feb 11 - 03:15 PM

Once on The Last of the Summer Wine (UK Telly Programme) Norman Clegg stood in Sid's cafe and asked -

"If I'm standing here, this is my Right Leg!
When I turn round - Why isn't it my Left Leg?"

That one should keep you all going for a while...
Quack!
GtD.

Subject: RE: BS: Reflection & the behaviour of light
From: GUEST,Chongo Chimp
Date: 04 Feb 11 - 03:56 PM

Beats me why the hell anyone wants an explanation for this kinda stuff. You already know what happens. Just deal with it. Do ya think any dog ever asked why somethin' happens? No. He just deals with it. That saves him wastin' a hell of a lot of time on nothin' at all, the way people do.

- Chongo

Subject: RE: BS: Reflection & the behaviour of light
From: Slag
Date: 04 Feb 11 - 05:14 PM

Down Chongo, DOWN! Surely you want to know your gun-hand from the other! That's two similar but separate functions and you DO want to keep them separate.

Subject: RE: BS: Reflection & the behaviour of light
From: GUEST,Willow
Date: 10 Feb 11 - 12:31 AM

Dear Mr Burke

Are you an ex-W4er ?

Subject: RE: BS: Reflection & the behaviour of light
From: The Fooles Troupe
Date: 10 Feb 11 - 03:27 AM

This as bad as the ranting on Bobert asking about burning 50 pounds of firewood - I didn't get involved - once burnt, twice shy ... :-P

Subject: RE: BS: Reflection & the behaviour of light
From: JohnInKansas
Date: 10 Feb 11 - 07:13 AM

Take the glove off your left hand.

Turn it other side out.

Put it on your right hand.

(It's harder with a left shoe.)

So why don't your pants go on backward if you turn them inside-out?

All that accomplishes is to make it harder to get to your ....


... (pocket change?)

John

Subject: RE: BS: Reflection & the behaviour of light
From: Bill D
Date: 10 Feb 11 - 11:42 AM

Ask Alice