Radiator Leak: Newsletter Challenge (02/20/07)

Thanks for the kind words, Fyz (not Fyz Ed),

This is dummy again and I need help because I'm having second thoughts about this polarization thing; it just won't resonate in my mind, if you will, so I can't quite get comfy with it. Seems to me that any thermal effect that might intervene between an actual, proximate water body and the observers, would also intervene between a virtual object at a boundary and the observers--such that no conclusive determination could be made by (constructive or destructive) polarized-lens viewing of either object (either a nearby real pool of water or a far distant real, or more distant virtual (i.e., reflected sky) pool of water).

So let me suggest something else as a possibility; something I have not seen mentioned as yet: interference effects.

Suppose our intrepid ATVers, instead of looking through eyeglass lenses, instead held the shades at arm's length (or, with mutual help, further), with the lenses in the palm and the ear pieces opened and pointed upward. Now imagine that the observer panned with the glasses--panned the two vertical ear pieces--side to side, from right of the hoped-for engine coolant to left of same, all the while noting the interference visible at the earpieces. One might expect that as the ear pieces passed in front of an actual, nearby body of water, the interference effects caused by the ear pieces would disappear--disappear because of the cooler (and evaporating) water and the cooler air overlying the water. On the other hand, if the seeming water was far distant water, or sky, and only a mirage, a reflection, of the actual water or sky, then the interference seen at the ear pieces would not disappear as they passed in front of the "water" image--since the thermal effects giving rise to interference, not being an actual, fixed object, would not also be transmitted to and reflected off the mirage-forming boundary. Any thoughts about this?

Or am I trodding into deeper, wronger quicksand?

I'm not quite certain what you mean by "interference" in this regard. In any event, doubtless there will be visual differences between the images that come from a water reflection and a mirage, and the presence of the earpieces may help to focus the eye. But (mid-afternoon) the distortion due to the intervening air means that it is almost impossible to discriminate between the position or colour of reflections via the the lake vs those via the mirage. Fortunately, the polarisation of the transmitted light is virtually unaffected by the shimmer, which is why the polarising sunglasses can be effective. The only problem would be if the light being reflected was already strongly polarised (either horizontally or vertically - we could cope with angled polarisation), in which case the change in intensity would be the same for the pond as for the mirage.

Fyz

Neither am I quite certain. What I was trying to envisage would be something like double slit interference--in the optics sense--except without projecting onto a surface(?). I was brought to mind of it by remembering summers here looking out through my house's iron gate entryway on a hot, mirage-conducive day when rising heat waves can be seen rising (unfortunately, though, no mirages in the field of view). Somehow, when seen through those bars (at opposed to through the opened gate) one can actually see the light waves--or, at least light, wave patterns--a phenomenon visible only as a consequence to the vertical gate bars being there. I would suppose that the light waves are being made visible as a result of constructive reinforcement brought about as a result of inteference by the parallel bars. So I was wondering, then, if there had been a mirage beyond those gate bars--such that, in addition to viewing scattered light from the distant sky and terrain, I was also viewing parallel oriented reflected rays from a mirage image--would the bars have an effect on how the mirage appeared--or didn't appear.

Also--and this might be yet another vein--I was tempted, in lieu of interference, to mention the word, birefringence, as in a quality of the sunshades lenses themselves. But then we would be coming back to polarization---I think(!?)

Of course, if the eyeglass earpiece conjecture is all bunko, and like Masu I was just playing devious tricks, I would have to say here: Sorry, the question we were looking for was, "What is 'grasping at straws?'"

Uhoh! Just got a thought impulse about light spectrum filtering....so I gotta go back up to that one.

I don't know what you are seeing as such, but it's far more likely to be some other sort of interaction than light waves. This could be an optical illusion, based on the eye's propensity to see patterns, or there could be another moving pattern that is sufficiently similar to the bars of the gate to cause moire effects. Obviously, I can't say, not having the evidence in front of me - but it is even less likely that you could sense anything of a light-wave nature in the presence of shimmer than that you could in its absence. (Unfortunately, the presence of water would not prevent the shimmer)

Yes, birefringence is a function of polarisation - but as polarising sunglasses are designed to block one polarisation rather than the other, it would be difficult to observe birefringence even if there is any there (except with a second polariser - but only the first polariser in the sequence will interact with the incoming polarisation). [In any case, birefringence is not exactly desirable in a pair of glasses].

BTW, in this context, I fear that 'grasping at straws' includes the way the visual cortex tries to impose patterns on everything, including random information (did you ever try looking for pictures of faces in the pile of a plain carpet?)

Have fun

Fyz

Masu, Jorrie

Curious to know your take on #54.

Ross

Hi Ross

I think Fyz, our 'resident physicist' has it pretty well covered in post (oops, #62, I think?) I concur with that view.

Regards, Jorrie

Go over there to determine if that substance is water.

it only took 5 seconds with the four wheeler to reach it.

KISS= Keep it simple stup.. , do not waste your brain power.

It is certainly water, since tom had noticed his radiator leaks.

My 2 cents,

A. Kimiadi

Problem with rushing mindlessly forwards is that if it is a mirage, it acts like the carrot before the donkey ..... drawing you ever forward and soon 5 seconds becomes much more than that.

Also, not sure about the logic on the last sentence. How does noticing something prove something else?

Red.

of course you should have side reference point such as a tree before going 5-second dash. Make sense.

Mirage sometimes cannot be filterred thru sunglasses. it depend on sunglasses quality.

2 cents for my thought, 3 cents for your thought.

Regards,

A. Kimiadi

My thoughts are that John and Tom are most likely North American names and thus are probably located there. Therefore, in the early afternoon, the image of water to the southwest is likely directly under the sun in the sky. The reflection of the sunlight on any water would be directly inline of their vision. This reflection of sunlight on the water would have some polarization. This image of reflection would be reduced through polarized sunglasses. A mirage would not be nearly as affected by the sunglasses do to the fact that the light rays from the sky are not reflected and thus polarized, but actually bent (refracted) along a narrow angle as they pass from cooler air to the hot air located next to the ground. This gives the impression of reflected light because so many of the light beams become aligned passing parallel to the ground.

Of course I agree with your conclusion - but I don't think the names really help - they could just as easily be Australian*. The US origin of the question may be significant. But perhaps the best justification for assuming that they are in the Northern Hemisphere would be that it makes this bit of data relevant, as it ensures the light from the sky is not strongly polarised horizontally, and so ensures the method will work.

*If it's the US, would Tom be short for Tomasina?

Fyz

Yes! but the mirage is not real, it's just a poo ping, so there you can find the water in the desert illusively.

If you take it as real something, you are wrong and find nowhere to back!

Its a obtuse question!

Well...I would suggest they both get on JOHN's four wheeler and go check it out.

As for the sunglasses, I would think that if Tom stands with his back to the image and looks into the front of his sunglasses he would see whether or not it was a mirage. Use them like a mirrow. A mirage wouldn't show in the glasses would it? It's a trick of the eye, when you're looking at a mirage....but you can't fool the sunglasses.

if its water, it will be surrounded by significantly more shrubs and trees than the rest of the desert and probably have a man-made berm most of the way around it.

The real water reflection will partially disappear by the polarized glasses rotation, and the mirage won't. The mirage is not a reflection, just a diffraction so the light is rotating polarized. Ant

"If the human mind were simple enough to understand, the human mind would be too simple to understand itself. " No such verb as to simple, eh? :D A

Cogito ergo sum, cogito?

Lungus Hock, Ergo Luggie Hock?

Were is past tense of the verb to be. Strife

Light reflecting off of water is linearly polarized. Light from mirages however are not polarized at all. Light from a mirage is bent from refraction, not by reflection. A layer of very hot air close to the ground is lower density then the air above, light is bent (refracted) away from the ground and toward the far away viewer. Since the light is merely sunlight that is bent, it is not polarized. If Tom were to hold his sunglasses between himself and the [apparent] water and rotate them, the intensity of the image of the water should vary if it is real do [due] to the polarizer on the sunglasses, if it is a mirage, the water will look the same no matter how the sunglasses are rotated.

So many fudge words (see underscored)--the best that could be said is that this method, depending on viewer skill and experience, might work (kinda) some of the time, and leave a viewer still puzzled the rest of the time. Consider also that a very hot air layer overlying actual water should be at higher elevation than the surrounding terra firma. Could it not then be surmized that incident light reflecting off actual water would include light scattered from sky which is higher above the horizon; and, thus, light of different hue (color intensity) that that which would be reflected at a lower incident angle from a rays coming from near the sky horizon. And could not this light from a higher boundary also be refracted towards a viewer, and also be of "different intensity" than the surrounding sky?

You are not using the correct "due". Your sentence that includes "... if it is real do to the polarizer...." should be "... if it is real due to the polarizer...". Your poor English skills are going to get you into doo-doo.

Wilfy

Brilliant solution next best idea would be to leave the busted four wheeler and check it out. strife

I knew I should have bought polarized sunglasses!

Tom dutifully takes the sunglasses and cautiously climbs atop and stands on the roof of his four wheeler to change the angle at which he is exposed to the mirage/water source. Subsequently, he pulls the sunglasses from his face and raises them high above his head, once again increasing the angle of view of the mirage/water source. Being several degrees above the horizon from his original perspective, he looks into the reflected picture on the lens of the sunglasses and the vision of the mirage either disappears or is enhanced by the clarity of rising above the refracted heat image of his new vantage point. The heat mirage will dissapear with a variance in height perspective; the water source will continue to be seen on the lens from this new perspective.

This is ridiculous. Only a fool would go into the desert without plenty of spare water and maps which show reliable water sources. Most real water in deserts if visible from a distance is transient in nature. Such water sources are almost always in low areas surrounded by deep gushy mud. In the deserts I frequent the price for rescuing someone who tries to drive his 4wd up to the water starts at $1000 for the tow job to get him unstuck. Dah Fish from CA

An oasis mirage is formed when light from the Sun heats the ground and the air just above it. A ray of light from the sky passes first through the cooler air high above the ground and then into the warmer or hotter air near the ground. The ray bends upward just before it reaches the ground. A person cannot see the path the ray takes. Since the ray is travelling upward when the person sees it, the person thinks that the ray came from a spot on the ground.

The conditions cited in the case is perfect for an oasis mirage. The apparent location is southwest where the sun is setting and it is early afternoon. It is a mirage. These guys only have themselves to blame for driving a poorly-conditioned vehicle and not bringing extra water.

Unless they have some way of repairing the leak in the radiator. It may not really make a difference. Even if it really is water it would just leak out again.

Much to my embarrassment: the proper answer is that, unless the images are visually distinguishable without the polarising sunglasses, polarisation will be of no help whatsoever.

The reason for my embarrassment is that it was not until I looked at CR4's reply (that implied that light reflected from water would be linearly polarised) that I looked at the numbers. This was because I assumed (wrongly, as it turns out) that the team would have performed the requisite sums. It's well known that complete polarisation only takes place near the Brewster angle, which corresponds to a minimum elevation of the source of 37-degrees above the horizon. So there's no chance of mistaking this for a mirage - or indeed of needing a vehicle to reach the water, unless they are at the edge of a cliff.
So, having spotted this howler, I did some numbers. Typical mirage reflections are limited to about 1-degree elevation, although they can reach 3- or 4-degrees elevation if the mirage is created by a curved dip. Working on an elevation of 2-degrees, the ratio between the proportion of light reflected from the different polarisations is 0.94:1 - and there is no chance that you can sense this in the presence of typical levels of shimmer that might occur in a desert..

Ouch

Fyz

Unfortunately, the polarizing angle for the air-water interface is 37° to the horizontal and drops sharply thereafter, which means that at about 1° to the horizontal which is the angle at which the traveller is observing the mirage, hardly any light is polarised and the wearing of polaroid glasses would hardly make any difference.

"do to the polarizer" is spelled "due to the polarizer".

What if you don't have polarized sunglasses?

Apparently Tom did. The result will be that they don't go over to whatever is causing the reflection, regardless of whether it's mirage or water, because there is no polarisation effect for mirages, and, for reflections from water at shallow angles, polarisation effects are tiny (see posts 120 & 122)

_______________

A little knowledge is a dangerous thing