Why Does Light Penetrate to a Certain Depth?

[nmsurobert]

someone asked me about this and I don't know how to answer them. I will attach a couple pictures.
They asked me why the light penetrates a certain depth then just disappears instead of gradually fading away. Particularly in the sand bed as seen in the pictures. I've read through this https://en.m.wikipedia.org/wiki/Penetration_depth already. So is it just a property of em waves to diminish quickly depending on the medium? I just took an optic class last semester where I was fortunate enough to learn close to nothing so sorry if it's a dumb question.

 

[BvU]

This is not light penetrating in the sand bed. This is light from the light tube above that reflects once on the glass-air interface.

[edit]sorry, armament is wrong word; I was looking for the word to translate dutch word armature (light source)

 

[nmsurobert]

This is not light penetrating in the sand bed. This is light from the light tube above that reflects once on the glass-air interface.

[edit]sorry, armament is wrong word; I was looking for the word to translate dutch word armature (light source)

Hmmm well i don't understand that. Why does that happen?
Light penetration or not, can someone explain why only about an inch of sand beneath the surface is visible and anything beneath appears to just fade to darkness?

 

[A.T.]

Hmmm well i don't understand that.

http://www.photonics.com/images/Web/Articles/2016/2/10/Eye_Light.jpg

 

[nmsurobert]

http://www.photonics.com/images/Web/Articles/2016/2/10/Eye_Light.jpg

Ah awesome. I've seen that before. Thanks.

Can you explain why only about an inch of sand is luminated and then drops to basically nothing ? Or is it explained in the wiki link?

 

[blue_leaf77]

Can you explain why only about an inch of sand is luminated and then drops to basically nothing ?

I hope this picture below will help

The dashed line marks the ray that is barely obstructed by the sand's surface, the point where this ray reflects the second time marks the start of the shadowy region.

 

[nmsurobert]

I hope this picture below will help
View attachment 104262
The dashed line marks the ray that is barely obstructed by the sand's surface, the point where this ray reflects the second time marks the start of the shadowy region.

Ooooh ok. That makes total sense. Thank you! Did you make that just to explain it to me? This forum is great.

 

[nasu]

This is not about light penetration. More likely the sand is stratified, with the bottom layer having a darker colour.
This stratification is mentioned in aquarium sites, like this one. See picture towards the end.
http://www.reefkeeping.com/issues/2003-03/eb/.

 

[blue_leaf77]

This is not about light penetration. More likely the sand is stratified, with the bottom layer having a darker colour.
This stratification is mentioned in aquarium sites, like this one. See picture towards the end.
http://www.reefkeeping.com/issues/2003-03/eb/.

If we take a look at the boundary between the light and dark region in the photos in the first post, the border shape follows almost exactly the shape of the sand surface that is exposed to the direct light. I don't think a stratification process can be the cause of such perfect congruence.

 

[jbriggs444]

So, rather than debating theory, how about an experiment? Take a piece of black plastic (for instance, a strip of electrical tape folded in half, sticky side to sticky side). Place it on the surface of the sand, flush up against the glass so that it blocks all light that would otherwise penetrate the sand. See if the sand below the tape is still illuminated. Compare with the sand to either side of the strip.

Edit: if the tape floats, weight it down with a dusting of sand as needed.

 

[OmCheeto]

So, rather than debating theory, how about an experiment? Take a piece of black plastic (for instance, a strip of electrical tape folded in half, sticky side to sticky side). Place it on the surface of the sand, flush up against the glass so that it blocks all light that would otherwise penetrate the sand. See if the sand below the tape is still illuminated. Compare with the sand to either side of the strip.

Edit: if the tape floats, weight it down with a dusting of sand as needed.

I observe the same phenomenon in my fish tank, and that worked!
Placed flat on the gravel, there was still illumination.
But placed flat against the glass, there was no illumination.
So the light was being internally reflected by the glass.

I also did some laser testing with a white cd, at and below the surface of the gravel.
I like blue_leaf77's illustration the best, as it best illustrates that the lit band is due to the non point source nature of the lamp.
With the laser beam, I was able to illuminate only the spot an inch below the level of the gravel.
Everything above and below that was dark.
Unless! I increased the vertical angle, and then I had multiple points illuminated, with diminishing intensity, similar to A.T.'s illustration.
But as BvU correctly stated in post #2, nmsurobert's band is due mostly to a single reflection.
Moving the fish tank lamp forward, will increase the height of the lit band of sand.

ps. My fish must think it's the weekend and time to party, as it's after 1am and the lights are still on. And that laser light show. Woo Hoo!

 

[nasu]

If we take a look at the boundary between the light and dark region in the photos in the first post, the border shape follows almost exactly the shape of the sand surface that is exposed to the direct light. I don't think a stratification process can be the cause of such perfect congruence.

Yes, I observed that feature and I found it interesting.
However stratification of sand is described by several sites about aquariums. And you can see more than one layer, with a darker one in between.
And it may have a similar behavior (following the features of the surface). If I had an aquarium I would just turn off the lights from above and check if this strataification exist or not, with a small light from the side.

 

[OmCheeto]

Yes, I observed that feature and I found it interesting.
However stratification of sand is described by several sites about aquariums. And you can see more than one layer, with a darker one in between.
And it may have a similar behavior (following the features of the surface). If I had an aquarium I would just turn off the lights from above and check if this strataification exist or not, with a small light from the side.

I have a fish tank, though as you can see from my image, it is filled with pebbles instead of sand. Some of the pebbles are somewhat translucent, so this complicates the matter. But with front illumination, there is no visible stratification.
In trying to understand the height of the lighted portion, I did the maths yesterday. It was very complicated, as I had to shift directions by 90° after the light hit the water.
Anyways, after much head scratching, my maths claimed that the light should illuminate the top 12.8 mm of gravel, based on angles, and the 5 mm thickness of the glass. So I dug my mostly opaque cd out of the trash, stuck it against the side of the tank at the bottom, and the top 13 mm were illuminated.

Nothing is drawn to scale, nor are the angles displayed correctly.
The angles only represent the relative change of direction of the light paths.​

ps. I've been an aquarist for about 50 years, and had never noticed this phenomenon before. Hence, my interest.