Reef Chemistry Question of the Day #13

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #13

If the water clarity is uniform, and 50% of the intensity of a particular wavelength of yellow light penetrates to a depth of 15 feet on a coral reef, what percentage of the original intensity penetrates to 45 feet?


A. 50%
B. 25%
C. 15%
D. 12.5%

Good luck.

 

[cheezybuda]

I WAY overthought this question. I was going through the attenuation coefficient of yellow light through different types of seawater.

I completely forgot that this is a simple Beer-Lambert law relationship.......silly Cheezy.

The answer is D 12.5%

 

[leptang]

I have no idea
I'll guess... D 12.5%

 

[cginter]

I'm gonna go with D

 

[beaslbob]

depends on the reflector

 

[Randy Holmes-Farley]

depends on the reflector

While there may be some coral reefs that are 45 feet deep that have reflectors, this one is a natural reef.

 

[reggaedrummin]

Well if the density doubles every ten feet then it would make sense to me that the wavelength penetration would be cut in half every ten feet so my answer is D, 12.5%

 

[Randy Holmes-Farley]

Congratulations! Folks did a good job with this one. The answer is 12.5%.

If every 15 feet cuts back the light penetration by 50%, then at 15 feet it is 50% of surface lighting.

After another 15 feet (30 total) the light drops another 50%, to 25% of the surface level.

After another 15 feet (45 total) the light drops another 50%, to 12.5% of the surface level.

Things can be more complicated in an aquarium where the lighting may be dropping with depth for reasons other than light absorbance, including the distance from a point source (if the tank has them) and reflection off the front and side glass.

 

[reggaedrummin]

I got the right answer by the wrong calculation... I consider this a win![emoji12]

 

[leptang]

So do you think a refractometer in poor light can give false readings?

 

[Randy Holmes-Farley]

So do you think a refractometer in poor light can give false readings?

By poor light, do you mean dark? Yes, if you have a hard time seeing the line.

As to color, that is trickier. Refractive index does hugely change with wavelength. The standard for visible light is always yellow (specifically, the yellow doublet sodium D line, with a wavelength of exactly 589 nanometers). The difference in refractive index for seawater and fresh water is the same as the difference in refractive index for pure water in blue light and in yellow light.

That sounds like a huge problem, but as long as you calibrate it with the same light that you use for measurement, the effects should be minimal.

That said, I've never looked into the color effect before, relative to the salinity effect, and the fact that the color effect is so large means that folks really do not to be certain to use the same lights for calibration as for measurement. I think I should post that as its own thread!