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Posted on 12/10/12 09:17:30 AM |
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Steve Caplin
Administrator Posts: 6997 Reply  |
The wet road problem
A few months ago I posted a Friday Challenge whose theme was to create reflections in a rainy street. At the time, I said I thought the reflections should smear downwards. Here's a photograph I took while driving on a motorway during a rainstorm (forgive the camera shake, it wasn't easy to do): 
Look at the reflection of the lights, and even the number plates, in the wet road. Can anyone come up with a convincing explanation as to why they should be streaking down like this? |
Posted on 12/10/12 10:49:37 AM |
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Deborah Morley
Makeover Magician Posts: 1319 Reply  |
Re: The wet road problem
Think it's the shutter speed. As it was taken at night on your (iPhone)? the shutter speed would have been very slow, I have an image where the light arcs, I'll dig it out. |
Posted on 12/10/12 10:54:59 AM |
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Deborah Morley
Makeover Magician Posts: 1319 Reply |
Re: The wet road problem
Found the picture, even more bizarre than I remembered!  |
Posted on 12/10/12 12:04:43 PM |
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Steve Caplin
Administrator Posts: 6997 Reply |
Re: The wet road problem
Not shutter speed - that's how I actually saw it! |
Posted on 12/10/12 1:41:29 PM |
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Jota120
Ingenious Inventor Posts: 2615 Reply  |
Re: The wet road problem
I don't think reflection comes into this too much, its the scattering of light, which of course simple reflections do. That wet road surface is so perturbed and with spay as well, but maybe spray lesser issue here, so scattering from the light sources is also perturbed when viewer sees outcome. A rainbow is beautiful example, but not so beautiful refraction is taking place here too in the spray. I'll give an example (beautiful) of light scattering on a perturbed wet surface. I have some examples too. P.S. try riding a Jota in the rain and cant see very much Aah!, especially in the headlights.... ! Well just my thoughts .....  |
Posted on 12/10/12 2:02:43 PM |
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Jota120
Ingenious Inventor Posts: 2615 Reply |
Re: The wet road problem
Made a couple of edits ... |
Posted on 12/10/12 10:15:46 PM |
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Paul 2007 thru 2010
Lego Legend Posts: 361 Reply |
Re: The wet road problem
In this case if it WAS shutter speed related then you'd need to ask yourself why are the cars themselves are not streaked. So it's fairly obvious that's not the reason. You could ask why, in a photo of an aeroplane, do the propellors look blurred. Could you answer "Not shutter speed - that's how I actually saw it"? That IS how you saw it, but (slow) shutter speed would still be the reason they are blurred. I think someone else mentioned this but I will add another vote to the unevenness of the surface of the water. A reflection in a perfectly still lake would be like a mirror (just like the images in the Links forum from the Sun Newspaper) one point in space would have one point in the reflection but as the water is not still, it's constantly reflecting points from different positions. As it was dark at the time all that can be seen is the lit up parts of the vehicles. It would be interesting to see the same photo taken in full daylight. I would assume the wet road would be reflecting EVERYTHING this time and nothing well defined would be noticed. |
Posted on 14/10/12 3:50:25 PM |
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tank172
ThreeDee Thriller Posts: 692 Reply  |
Re: The wet road problem
I think it's a bit of perspective and optical illusion. The light is reflecting off the water on the road, and the reflections are heading towards the camera. ... instead, the light reflections appear to run straight down vertically. Same with the sunset pic, we would expect to see the reflections running with the perspective of the water plane, but instead, they 'appear' to run straight down. Not sure if it is correct, though.  |
Posted on 14/10/12 6:05:50 PM |
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Deborah Morley
Makeover Magician Posts: 1319 Reply |
Re: The wet road problem
According to 'The Theory of Reflection' the angle of reflection is the same angle (but going in the opposite direction) as the light hitting, in this case the wet road, so as the lights are at 90degrees to the road the reflections are also 90 degrees, but going the other way. I think that as the cars are moving and causing ripples in the surface water each ripple is causing a reflection and as the angle of reflection stays the same you end up with a long line. That's my theory and I'm sticking to it! |
Posted on 14/10/12 7:26:54 PM |
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Jota120
Ingenious Inventor Posts: 2615 Reply |
Re: The wet road problem
Agree with you Deborah, especially your second paragraph. At least you made it clearer than me. To be precise the the angle of incidence = angle of reflection, but when the angle is" too deep" it does not get reflected, its just refracted (goes inside to feed the fish and vegetation.....  .)
At the same time also thinking to a smaller degree the spray, 2nd order. When the spherical-ish spray will also reflect light by internal reflection/refraction. But again not the most significant effect? Example Cats Eyes But more, example the simple case without spray:  |
Posted on 15/10/12 07:57:59 AM |
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Steve Caplin
Administrator Posts: 6997 Reply |
Re: The wet road problem
So the simplest explanation seems to be that it's curved water drops on the road that reflect objects behind them, leading to streaked reflections further down the road. But in this case, why are reflections only elongated downwards, and not to the sides? Thanks for the new photo, Trevor, which I think shows the issue more clearly, but I'm still not sure why the effect works in this way. |
Posted on 15/10/12 11:13:45 AM |
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Deborah Morley
Makeover Magician Posts: 1319 Reply |
Re: The wet road problem
I think if you say that there are multiple puddles, that would have the same effect as ripples on water. See note below.  |
Posted on 15/10/12 1:50:48 PM |
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Paul 2007 thru 2010
Lego Legend Posts: 361 Reply |
Re: The wet road problem
I'd have assumed the reflections were streaked towards the observer. Is this not the case? |
Posted on 15/10/12 2:11:56 PM |
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Steve Caplin
Administrator Posts: 6997 Reply |
Re: The wet road problem
Deborah, that's interesting, but does it fully explain the phenomenon? At 80mph, driving in the rain with one hand on the steering wheel and the other holding my iPhone to the windscreen, I didn't have the chance to measure the direction of the ripples! |
Posted on 15/10/12 2:25:04 PM |
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Eggbox
Ovoid Opportunist Posts: 797 Reply  |
Re: The wet road problem
"At 80mph, driving in the rain with one hand on the steering wheel and the other holding my iPhone to the windscreen, I didn't have the chance to measure the direction of the ripples nor yet the presence of the police car but I can assure you it was all in the pursuance of scientific observation M'Lud!!"  |
Posted on 15/10/12 7:31:41 PM |
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Jota120
Ingenious Inventor Posts: 2615 Reply |
Re: The wet road problem
Maybe a clearer way to explain it is the uneven ground and water are perturbed by either the substrate (e.g. my last image) or the cars perturbing the the water surface, like the wind. I guess the key point is the surface is acting like a mirror, but it is not flat, like the waves on the sea and reflections of e.g. sunset. Water acts like a mirror when get critical angle of reflection/refraction caused by the difference in the speed of light through different media. It goes faster in air than water, hence reflection at the critical angle. See those great sunset photos for example. Light only goes in straight lines, so in this case from the source to the observer. Since we, the observer only see the reflection of the source on an irregular "mirror" with range of horizontal inclined surfaces, we get a many viewed inclined mirrors, but each is a reflection with light going straight from source to viewer, but only when critical angle or above is correct. Maybe I should add a diagram later?..a picture tells a thousand words Something quite different, with airborne water/mist, the light gets scattered but by internal and external refraction and by neighbours, just like we see in clouds, mist and fog. You can see the scatter in the air above the cars in your image and my last one by the street lights. So then it get complicated when have the two forms reflections and scattering. There is/are Wikipedia entry, but I don't think it is very helpful here. PS, BTW, yes the reflection of light does go all around, for example another viewer in a another car or sidewalk and the view different then. But the reflection only goes from source to viewer and the perception there of. When you look in a mirror you only see your view, but many have other views i.e. the photons go and are reflected everywhere, but a viewer has just one view. Works with the Hubble too, or if get an array telescope with multiple views, get a lot more information, the multiple view! Trevor |
Posted on 15/10/12 9:37:26 PM |
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GKB
Magical Montagist Posts: 3972 Reply  |
Re: The wet road problem
I have only just seen this thread as I am busy trying to keep dry in Italy! ... and failing!!! Basically Deborah and Trevor's solutions are correct. If you have a scene being reflected in perfectly still water or a mirror then you will get a perfect reflection (but see later in my thtread). Once you introduce some kind of surface roughness with a water covering that conforms to that surface then you will start to have multiple mirrors and the reflection will appear distorted. If you now take an infinitely small point light source and reflect it, it will send out light rays in a straight line which will be consequently reflected in a straight line. As it is not physically possible to have an infinitely small point source of light then there will be a certain width to the light source due to photons being emitted across the width of the light source. This will create multiple straight lines which will result in multiple straight lines being reflected and give width to the reflection. If you stand, say, four people around this light source they will all see the straight lines of light being reflected towards them but, for any given individual sets of photons being emanated, they will see different shapes to the reflection as the reflections are coming from different 'mirrors' i.e. the surface of the road. If you change the height of the viewer then you will see different lengths to the reflection as the angles of incidence change. What you might not realise is that you will actually be seeing TWO reflections of the scene but one is much darker. This is due to part of the light beam being reflected from the upper surface of the water and part of it penetrating to the lower surface where it is in direct contact with the surface of the road. Much of this light will be absorbed by the road surface. The amount depends on the colour of the road surface. It is for this reason that reflections are always darker than the light source and is another reason why The Sun's photographs of 'reflections' in the Thames were fakes. It is this phenomenon that requires mirrors to have a silvered coating on the rear surface. You won't see the reflection from the front of the glass because you are looking at it along the 'normal'. But if you have a little surface roughness or a surface that is not parallel to the silvered coating you will start to see double reflections. Double reflections can make the reflected image appear soft. For some uses front silvered mirrors are required to cut out any possibility of this internal reflection. Hope you all follow that 
Gordon _________________ You're never too old to learn something stupid. |