General Question

Ltryptophan's avatar

How does this aspect of visual perspective work?

Asked by Ltryptophan (10241points) February 26th, 2010

Say you are looking at the calm reflective surface of a pond, and as you walk by its bank the moon’s reflection follows you. At every step of your path the moon’s reflection has changed position on the water’s surface, but presumably it would still look the same from the other positions on your path. Why doesn’t the moon look like a stripe then? Is it verysmall differences in the surface? What is happening here?

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27 Answers

lilikoi's avatar

I’m sure that it has something to do with the fact that the moon is very far away, and how your eye perceives the light. I don’t think the specific phenomenon you describe has anything to do with the surface. Oh here is an explanation that seems reasonable. Great question.

ETpro's avatar

Light reflected from the moon follows a straight path while it travels through a given medium. The partly reflective surface of the water bounces some of the light along the same path in one dimension, but at an equal and opposite angle and up to your eye. For the moon to smear across the water’s surface as you move, the light would need to ‘remember’ where you first saw it and hit there as well as at all other points up to your straight line of sight defined by a single plane through the moon, the reflection on the pool’s surface and your eye. To do that, light would have to bend.

Ltryptophan's avatar

but, i mean if all those spots are seeing bright? @ETpro i think you explained it, I just don’t comprehend it.

Ltryptophan's avatar

@lilikoi that link seems to say that the whole pond is lit up at once, right?

lilikoi's avatar

Yeah…. I mean I don’t see how the light could only appear in one place and not everywhere, seeing as the source is so far away. I think it does indeed appear everywhere, that’s why a night w/ full-moon is so much brighter (that you don’t even need a flash light in what should be pure black) than new-moon nights. But I think what ETpro must be talking about is how the human eye detects the light. Although he confused me, too.

lilikoi's avatar

Cuz you don’t see that bright line of light from all the other points along the pond from your one single position. As you move, the light moves with you, which means it has something to do with the angle that you are looking at the light water.

Ltryptophan's avatar

mind boggling…

lilikoi's avatar

yes! where is ETpro?? he needs to come back

Ltryptophan's avatar

I mean, I get what you are saying, but it’s counterintuitive or something.

Ltryptophan's avatar

mirrors don’t seem to work this way…do they???

Rarebear's avatar

Look at it this way. Your eye is like a movie camera, constantly taking pictures. It takes a picture, sends it to your brain, and then takes another picture. So it’s like you walked along the lake taking photo after photo, and so you see one moon.

If, however, you were to take a camera and hold the shutter open and walk with it, then you’d see a streak. But since your eye and brain don’t act like that, the moon stays resolved.

lilikoi's avatar

Yeah I agree with you @Rarebear but I still don’t understand why a single line of bright light is what you see at any given point.

@Ltryptophan I do think that if the surface of the water was replaced with a giant flat mirror, you would have the same effect, but perhaps a much crisper line as there would be no surface imperfections due to “chop” (wind swell in the water).

Ltryptophan's avatar

Why a streak, why not a giant blotch of white where the pond is? Shouldn’t the moon picture be on every reflective part of the pond? If I were to focus on a small area of the pond, or drain all but one spot…still the moon…so shouldn’t the brain be sorting a giant white reflection?

lilikoi's avatar

Oh I think I got it!

Perhaps some of the light reflected off the surface of the moon is dispersed widely while the moon itself also acts like a single source of bright light so the bright line we see on the water is actually the reflection of the moon itself from our low perspective on the pond edge at ground level.

Because look at this photo vs this one…. or maybe that is obvious.

lilikoi's avatar

I think if you were to rig a cable from one end of the beach to the other and move the camera slowly in one dimension parallel to the beach on a super long exposure, you would end up with a photo where the surface of the water is totally reflective. I think that’s what Rare meant when she said streak. I think you guys both mean the same thing.

Ltryptophan's avatar

that second picture, how can there be any dark spot????? WTF

Ltryptophan's avatar

this is my logical problem. Light=visible, Light hits water, water is reflective everywhere on its surface, light only shows on certain angle regardless of the fact it is clearly existing on all parts of the waters surface.

lilikoi's avatar

I see where you are stuck. I understand what is going on here just enough that my brain accepts the observations as logical, but not well enough to explain how it works, if that makes sense. I am gonna dig around the internet and see if I can clarify it.

Ltryptophan's avatar

I know that the problem is my understanding, I don’t think this should be so mysterious to me(how embarassing)!

Rarebear's avatar

@lilikoi In your two photos, if the water were perfectly still, you’d see a reflection of the moon as if it were a mirror. But since it’s not, you see the streak.

lilikoi's avatar

@Rarebear Oh, true, I agree with you. Then, if you were to use a mirror instead of water, you’d just see the reflection of the moon in the mirror. But how would the angle at which you are viewing the mirror change what you see?

ETpro's avatar

@Ltryptophan and @lilikoi Indeed, the light reflected from the moon shines on the entire pool unless something blocks part of it. But the light you see as a bright spot on the pool’s surface is light that is following a straight plane defined by the moon, the spot of light on the pool, and your eye.

You would get the same thing if you switches on a floor lamp on the other side of the pool at night. It would light up the whole pool area, but if you stood on the opposite side of the pool from the lamp, you would se a single reflection of the lamp. Look toward the far end of the pool, and you would still see the pool lit by the lamp, but no direct reflection. You would then just be looking at scattered light from the lamp.

lilikoi's avatar

@ETpro Yes, intuitively I know you are right, but I still can’t quite wrap my mind around why the light appears brighter in the plane formed by you and the moon than everywhere else. It must just simply be that more light is reflected to your eye in this plane (shortest path between you and the moon) than any other path available.

lilikoi's avatar

Oh yes, I understand now what you were saying before about light having to bend if it were to brighten up the whole pond.

The moon acts as a point source. We can assume for simplicity sake that all light originates from the moon. Like @ETpro said before:

Light reflected from the moon follows a straight path while it travels through a given medium (at this point the medium is the atmosphere). The partly reflective surface of the water bounces some of the light along the same path, but at an equal and opposite angle and up to your eye (see the “How Light Works” image here). Since the light is coming from a single source, and it follows a straight path, the light is radiating out from the moon in an infinite number of directions. Not much of the light will be reflected to your eye from most directions compared to the amount of light that is reflected to your eye where you and the moon are in the same plane. Because it is in this plane that the most light is reflected to your eye, there appears a bright line of light…. Something like that.

ETpro's avatar

@lilikoi Because light travels in the shortest possible route. A straight line to the reflective surface of the pool, and in the same plane up to your eye from the pool is the shortest possible route. The other light you see reflecting off other areas of the pool is much dimmer than the reflected moon because it is scattered light coming from reflections off irregular surfaces the light is hitting.

Ah, you got it while I was writing this. And explained it so well. Thanks.

lilikoi's avatar

Exactly, yep I get it now. Thanks so much!

Ltryptophan's avatar

Yes, I also understand now thanks for the help guys!

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