# Can a substance refract light (1 Viewer)

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#### Nacho

Can a substance refract light to such a degree that the angle the light is bent to be an acute angle?

The reason I ask: Take for instance Moon shine (not the kind that you drink). The answer you most often get is that it is sunshine reflected off the Moon, to your eye. That can't be the correct answer .. reflected light retains the image of the "source" (meaning emission/refraction) of the light. If it was truely reflected light we would see an image of the Sun. Rather, it is light from the Sun refracted from the Moon.

But, in this instance, how does the light get refracted to such a degree? Take the full moon .. that angle I talked about above would have to be virtually 0 degrees. Books I've read about refraction alway show (as examples, now) that angle being an obtuse angle.

Maybe the light undergoes multiple/many separate refractions on the surface of the Moon before it gets back to your eye??? Or maybe images can also be formed from scattered light in addition to refracted light???

[edit .. I guess I said this wrong. It is diffraction that causes the images, not refraction. Still wondering though about the process by which the light can get sent back to an observer at such a small angle, without reflection]

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#### Doc Al

Mentor
Re: Images

Originally posted by Nacho
The reason I ask: Take for instance Moon shine (not the kind that you drink). The answer you most often get is that it is sunshine reflected off the Moon, to your eye. That can't be the correct answer .. reflected light retains the image of the "source" (meaning emission/refraction) of the light. If it was truely reflected light we would see an image of the Sun. Rather, it is light from the Sun refracted from the Moon.
No, it's still just reflected light. It's doubly reflected: it reflects off the moon, then off the earth. You will only get an image if you reflect off a smooth surface like a mirror. Reflection off a rough surface (like the moon or the earth) will produce diffuse reflection---the light is reflected every which way---so no recognizeable image is formed. This phenomenon is all around us: When I look around the room right now I see all sorts of objects. (Physics books, mainly... ) They are all illuminated by the diffuse reflection of the light from the lamp. I only see the lamp's image if something's particularly shiny.

#### HallsofIvy

originally posted by Nacho
Rather, it is light from the Sun refracted from the Moon.
What in the world does "refracted from" something mean[/b}? "Refraction" is the bending of light rays as they pass through something.

IF all light were reflected at the same angle (as from a mirror), you would see an image of the original object. If the reflecting object is not smooth, light is reflected at different angles with the result that the light entering your eye coming from a small area of the reflecting object came from different points on the original object. The light depends more on the reflecting object than upon the original light source.

#### Nacho

Doc Al,

... This phenomenon is all around us: When I look around the room right now I see all sorts of objects. (Physics books, mainly...) They are all illuminated by the diffuse reflection of the light from the lamp. I only see the lamp's image if something's particularly shiny.

Yeah, I agree. The light could be reflected, or scattered, but after the reflection/scatter and leaving the surface of the object, wouldn't the light also have to undergo diffraction around [[obsticals/imperfections]] in the surface for us to be able to see that object VS the illuminating object?

#### Nacho

HallsofIvy,

What in the world does "refracted from" something mean ...

I guess you missed my edit at the bottom of the post. I corrected refraction to diffraction, but the post had been on the board for a bit of time before the correction. I didn't want to alter the original stuff written, but leave that as addenda in case someone had read the post before.

I have a book by Mark P Silverman, "Waves and Grains". There is a chapter on image formation, and he says images are formed by diffraction, and at root is an interferometric process. I can pull out some quotes out of the book if you want.

#### Integral

Staff Emeritus
Gold Member
Every thing we see is due to the defraction pattern that exists at the focal plane of our eyes, that is the focal plane in FRONT of our eye the image this plane represents is formed at the focal plane behind the lens. A lens is a form of analog computer, it transforms diffraction patterens into images. The difraction patteren is the superposition of all light passing through the focal plane.

The moon scatters light a certian amount of that scattered light passes through the focal plane of your eye, therefore you see the moon.

#### Nacho

Integral,

The diffraction pattern has to occur at the object though, is that correct? The eye (and lense) just transforms that into an image?

If that is correct, then what is the diffration from? Imperfections in the surface?

#### Doc Al

Mentor
Originally posted by Nacho
The light could be reflected, or scattered, but after the reflection/scatter and leaving the surface of the object, wouldn't the light also have to undergo diffraction around [[obsticals/imperfections]] in the surface for us to be able to see that object VS the illuminating object?
No, I wouldn't think that diffraction effects at the reflecting surface are needed to explain how a rough surface produces diffuse reflection which causes us to see the reflecting object instead of an image of the light source. (But I'm no expert on physical optics.)

#### Nacho

Ahhh, a light bulb may be finally coming on here! I'm questioning what happens at the object, not our eyes. And was thinking there was two completely separate processes going on with the light on the surface of that object: (1) reflection/scatter, and (2) diffraction.

This might sum up my question, if it is true. Is this a fair, maybe not entirely accurate but fair, statement:

Reflection is (I'll use the word) "coherent" diffraction.

#### Integral

Staff Emeritus
Gold Member
Originally posted by Nacho
Ahhh, a light bulb may be finally coming on here! I'm questioning what happens at the object, not our eyes. And was thinking there was two completely separate processes going on with the light on the surface of that object: (1) reflection/scatter, and (2) diffraction.

This might sum up my question, if it is true. Is this a fair, maybe not entirely accurate but fair, statement:

Reflection is (I'll use the word) "coherent" diffraction.
I would have to say No. A body emits light, photons are incident on a surface are adsorbed then re-emitted, there is no diffraction in this process.

At a point in "free space" all light energy passing through that point combine to form the difraction pattern.

#### Nacho

Integral, you wrote:

I would have to say No. A body emits light, photons are incident on a surface are adsorbed then re-emitted, there is no diffraction in this process.

Since I struck out there ;)

let me approach it another way. I'm confused on that, because it seems to me like that uses a quantum description for reflection, yet a somewhat classical description for diffraction.

(BTW, I'm not stating this below as fact; its just my understanding, and it's certaintly not complete -- that's why I'm asking questions)

Take a crystalline structure, and shine some light on it. In its shadow you might be able to see fuzzy edges due to diffraction.

Now, bombard that crystalline structure with X-Rays. The fuzzy edges in the shadow would not be as near as distinct, since the wavelength (and thus energy) of the illumination is also expressed in the diffraction pattern. Now consider the inside of the structure. It will produce a diffraction pattern also, which can be used to recover the atomic structure.

Whittle that crystal down to one layer and that should turn into reflection, with the wavelength of the radiation unrecoverable from the pattern.

What difference is happening inside that structure to produce the reflection VS the diffraction?

The diffraction pattern is not recovered only on the bottom side (the side away from the source of illumination) of that crystalline structure, is it?

Is is possible that the light is first diffracted (bent) by close approach to atoms within the stucture, on 1 layer say, and then is reflected from another layer?

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#### pallidin

"Is it possible that the light is first diffracted (bent) by close approach to atoms within the stucture, on 1 layer say, and then is reflected from another layer?"

Light is not "bent" on approach of an atom if the atom has electron shells. Special situations occurs during nucleus interaction, but not in this case.

A photon is never actually reflected. It is absorbed,by the electron in this case, and the electron rises in energy level, the electron quickly "falls back" in energy level, and a photon is emitted, sometimes in the same direction it came from. This gives the illusion of pure reflectance.

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#### pallidin

Ooopps, I meant 360.

#### Nacho

pallidin,

Light is not "bent" on approach of an atom if the atom has electron shells. Special situations occurs during nucleus interaction, but not in this case.

OK. How would you define diffraction then. On this same level. Integral said what was happening with reflection on that level, more of a QED level .. it agrees with what you said, and I agree also. But the descriptions of diffraction have been more on a "classical" level. I was just wanting the definitions put on the same level (and then there were other question in that post too).

How would you (and Integral) define what is happening with diffraction, on the same level as what you defined/said what was happening with reflection?

#### pallidin

Some thoughts: Diffraction of a single photon, I would suppose, is not possible, because the diffraction process relies upon the idea of multiple wave lengths of which to "split" into separate paths.
Given this, I would conjecture that the definition of diffraction is the lateral separation from a common source of multiple photons of differing wavelengths.

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#### Nacho

pallidin,

And then how far could you expect the transit angle of the diffracted photons/light to be deflected. I mean, could they be deflected 180 degrees, back to where they came from?

#### Nacho

I found the answer I was looking for, and it's really simple. I think the problem came from the terms "reflection/diffraction". They have both general definitions (GD) and specific definitions (SD).

Light from a source reflects (GD) off of the object. If it is specular reflection (SD) then all the light waves are in phase and will have traveled the same length when an image is formed in your eyes. There would be no interference; the result would be an image of the light source, not the object the light was reflected (GD) off of.

If it is diffuse reflection (SD) then the light waves would have traveled different lengths when the image is formed in your eyes. That is diffraction (GD) as each point on the object essentially acts as a point light source. That results in a interference pattern at the point in space it is viewed (by your eye) (really, all points in space). The result would be an image of the object rather than the light source.

Reflection (SD), diffraction (GD), and interference all play a part in the formation of the image of an illuminated object.

Thanks to everybody for your help.

(diffuse reflection being "scattered" reflection, scattered about because of a surface that is not completely smooth. If the surface was smooth, it would be specular reflection).

#### Nacho

This is an interesting phenomina I observed one time, and I've never been able to pull out the defining cause/physics behind it. I had never seen it happen before, and have never seen it happen since.

A few years back we got about a 4" snow during the day. That evening another cold(er) front moved through. The air was perfectly still, and it was very cold, and very humid. It was such that I would have thought if it had been a bit colder, moisture would be freezing out of the air!

I went out about 11 o'clock and drove up to the Toot & Totem to get a cup of coffee. We have amber colored street lights here, and as I would approach one, a beam of the amber light would shoot out from the street light all the way to the hood of my pickup. The beam of light wasn't straight; it was curved downward, bowed. It was around Christmas, and looked almost like a tinsel coming down from the street light to the hood of my pickup, being bowed because of gravity. The beam of light wasn't broken at any place, it was complete at all points along the beam, but was kind of "fuzzy" and "sparkling".

As I would approach the street light, the beam would shift the direction it was pointing, so that it was always tied to the street light and the hood of my pickup. I would drive by that street light, that beam go away, and then a beam from the next street light would extend down and touch the hood of my pickup.

It was a very pretty sight, surreal, and I wondered how it was doing that, but I was caught up with the sight and didn't have the presence of mind to stop and get out of my pickup and try to examine it. It might well have disappeared if I had gotten outside.

Since then I have tried to reconstruct what was happening, and can't quite pin it down. There are some other factors that could have played a role in its formation:

1) I had just taken my glaucoma eyedrop medication, and it might have not been completely dry, effecting my cornea. I doubt this had anything to do with it, because I was gone about 10 minutes, and the same thing happened on the way back home. I've never noticed any effect of the medication lasting that long after taking the eyedrops, and didn't notice any other sight effects.

2) I smoke .. not much in my pickup, but a thin film of smoke always collects on the inside of the windshield. The film is mostly transparent, but if light hits the windshield at just the right angle, the film turns translucent.

The weather had to be at least a main factor, if not the complete factor. And the smoke film a secondary factor, if a factor at all. If the smoke film played a role, the curvature of the windshield might have been responsible for the bow in the beam of light; refracting on the smoke particles along a curved surface. I doubt that, because the point of contact of the beam of light was outside the windshield, on the hood of my pickup. Also, I think the curvature is backwards for the effect.

I think it was due to refraction and reflection caused by water (maybe ice) molecules in the air -- essentially I was seeing an inverted rainbow (I think they are called "anti-solar point" rainbows, or something like that). I didn't see a "normal" rainbow also, because the street lights are directed downward only. The curvature or bow of the beam being the same for rainbows. There was only 1 color to the rainbow, because the street lights (and globes) transmit light in a narrow range of frequencies .. there being no different colors for the refraction to split out. Why it was "fuzzy and sparkling" .. I can only guess that maybe it was due to diffraction caused by the smoke particles on the windshield. I guess it also could have been caused by the water molecules actually being ice suspended in the air, and frozen with irregular shapes. The rainbow description doesn't make complete sense to me, because the beam of light was connected to the light source. I would have thought the "anti-solar" points would have been a distance away from the light source.

Anybody else want to give it a shot .. the physics that caused this?

[[edit -- found this fantastic site about Atmospheric Halos http://www.meteoros.de/indexe.htm . Mine might have been a Subparhelia http://www.meteoros.de/arten/ee45e.htm , the sprite going down towards the bottom, but there were major differences .. my beam was colminated, distinct, and of equal intensity at every point along the path. And it was much closer!!]]

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#### pallidin

"As I would approach the street light, the beam would shift the direction it was pointing, so that it was always tied to the street light and the hood of my pickup. I would drive by that street light, that beam go away, and then a beam from the next street light would extend down and touch the hood of my pickup.

It was a very pretty sight, surreal, and I wondered how it was doing that, but I was caught up with the sight and didn't have the presence of mind to stop and get out of my pickup and try to examine it. It might well have disappeared if I had gotten outside."

Nacho, I must say that you have a keen sense of observation and engagement with phenomenon. Great job! I do this all the time. You would make one hell of a theoretical physicist.

Anyway, the phenomenon you described, I have also seen. You would be correct to suggest that it would not be apparent outside your truck unless you wore very scratchy glasses which hand multiple concentric swirls from cleaning too much in a circular motion with abrasive tissue paper.
You see, this is an optical effect. It can also happen if your winshield is similary scratched.
I actually first noticed this phenomenon from observing the light reflected off a circular stainless steel stove-burner cover. It had thousands of concentric circular grooves on one side, very small in height, so I assume they were produced during manufacturing polishing.
In any event, whichever way I moved, a beam of light was very distinct coming from the center of the disk, to the rim, and always followed my movement. Of special note here, a second "beam" also shown in an exact 180.
Anyway, to see this right now for yourself, take a CD, turn it so that the shinny side is up, and a point light sorce in the distance(overhead light bulb) will do the same thing.

#### Nacho

pallidan,

Anyway, to see this right now for yourself, take a CD, turn it so that the shinny side is up, and a point light sorce in the distance(overhead light bulb) will do the same thing.

I did this, and it produces a very similar effect (less the color), but I don't understand the application to the windshield .. are you saying take one of the circles, straighten it out, and then apply that and many of them parallel on the windshield, kind of like a diffraction grating? Or just have the concentric circles on the windshield?

Anyway, you are absolutely correct in this!

I know this whole thing is going to sound concocted, but this same thing happened just a bit ago, good weather though. It must be a fairly common occurrence and I never noticed it .. I was looking for it this time. It probably registered so strongly with me before with the street lights because of the color, and being at night, and its appearance as sort of a Christmas tinsel decoration at Christmas time.

Anyway I was going out again to get me a cup of coffee. I was driving south, 4 oclock local time. The Sun was up from the horizon about, I guess 35-40 degrees, and south of west about the same I guess. I could see a beam of white light coming from the Sun, doing the exact same thing, but this time hitting the hood on the passenger side. I immediately pulled over and checked it out (people must have thought I was crazy!)

I got outside, the beam disappeared, just as you wrote. I got back inside, the beam was back. I took off my flannel jacket and put some spit on it (yuck! I should have used a bit of coffee!) and cleaned a spot of cigar smoke off of the windshield. I couldn't see a bit of difference in the portion of the beam transversing that "hole in the smoke". I then started my wipers and windshield cleaning fluid on the outside. It didn't have any effect initially, but my wipers are kind of old.

There was an arc in the windshield though, where the driver's side wiper had cleaned it good on the outside portion of its arc, and where there was some dirt on the outside of that arc. The windshield wasn't that dirty though. I moved around inside my pickup so I could view the sprite as it transversed that clean/dirty arc. When it transversed to the clean side, the sprite got a whole lot dimmer, but never quite went away.

So, the curvature of the windshield causes the curvature in the sprite, and dirt on the <b>outside</b> of the windshield causes .. diffraction .. I guess (kind of like a diffraction spike in a Newtonion telescope with spider vanes holding the secondary mirror). I would not have guessed the windshield the cause .. as I said I thought it was the ice/water/weather. You were right.

It must be a common occurence .. and was just so striking that night I saw it before. This time it was rather faint, and white colored. I bet I've seen it before in the daytime and didn't pay any attention to it. Still though, I have not noticed it anymore at night. Maybe the weather that night was depositing a fine mist ofmoisture on the outside of my windshield that I didn't notice, and that was taking the place of the dirt this time, or making it more pronounced.

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