A picture puzzle about absorption, scattering, transmission and reflection of different wavelength ;)

[Kai]

 

I took this pair of pictures from a Loire bridge (France) some days ago. The sun was already quite low on a clear sky (2 hours before sunset).

I find the UV picture exciting because of the phenomenon that almost no shadow of the bridge can be seen on the water.

 

Why is that?

 

 

Here is my explanation for the "missing" of the bridge shadow in the UV image:

 

In the UV image, only the scattered light from the sky can be seen on the right side (water). More or less depending on the angle. At the top of the image - at low angles - nearly all light is reflected (total internal reflection, (D)). Below, with almost perpendicular incidence of light, nothing is reflected anymore (F).
The direct light of the sun is not backscattered there, so it has no part in the picture. So there is practically no shade there.
Possibly the organic part in the water (containing chlorophyll) contributes to this.

In addition to the scattered light from the sky, sunlight is also scattered back on the sand ((A) and (B); sunlight plus skylight). Thus, the shadow can be seen in the left part of the image ((C), only skylight.

 

To me this image shows a beautifully complex interplay of absorption, scattering, transmission and reflection from different wavelength ranges and different substrates. I only miss diffraction phenomena here ;)

 

Certainly more can be said here (intensity differences, colors in the VIS image, ...).

What is your opinion?

 

 

 

[colinbm]

I look forwards to the answers...

[photoni]

absorption and reflectance percentages

[enricosavazzi]

57 minutes ago, photoni said:

absorption and reflectance percentages

I agree that is the key, with the addition of differential (wavelength-dependent) scattering, which is indeed related to differential reflection. If VIS comes mostly from the disc of the sun (as it does in the absence of clouds and haziness), the bridge will produce a sharp shadow in VIS. If UV comes from all over the sky rather than the sun disc (because of atmospheric scattering), it will produce little or no sharp shadow in UV. Because of shorter wavelengths, UV is more subjected to scattering than VIS.

[dabateman]

Clearly in the UV frequency dimension,  the bridge passes through an alternative space into a different dimension,  un seen by us visible wavelength trapped beings.

The camera, able to transcend these frequencies, gives us a glimpse into the alternative reality. 

Now we need to write a fantasy novel about this, because there is no science behind these ideas to make them science fiction. 

[Stefano]

If you look at the difference between the red and blue channels from the visible light image, you can see what the trend is at shorter wavelengths. Essentially the sky is brighter in UV.

[Andrea B.]

How long was the UV exposure time? I was wondering if there is any "smoothing" effect from the flowing water?

 

 

[Kai]

6 hours ago, Andrea B. said:

How long was the UV exposure time? I was wondering if there is any "smoothing" effect from the flowing water?

 

 

UV: Canon 6D-FS, Canon EF 2.8/28 mm, ZWB2 plus TSN575, f 4.5, 1/15 s, 3200 ASA; Location: Les Rosiers-sur-Loire, France; 2022-07-12, 19-38-22
VIS: Samsung S7

Here a contrast-enhanced detail. It shows faint waves on the surface.
The picture appears to be slightly blurred horizontally but the flow seems to be optically freezed...

 

 

 

[Kai]

17 hours ago, Stefano said:

If you look at the difference between the red and blue channels from the visible light image, you can see what the trend is at shorter wavelengths. Essentially the sky is brighter in UV.

Yes. Attached are the three color separations compared to the UV image.
The sky is blue (and violet/ultraviolet). The water appears correspondingly blue in the VIS image. the shadow of the bridge too.

[Andrea B.]

Thanks for the additional info.

This is really fascinating!