MadSci Network: Physics

Re: Does moonlight penetrate water to any depth?

Date: Tue Apr 8 15:36:26 2003
Posted By: Adrian Popa, Director Emeritus, Hughes Research Laboratories
Area of science: Physics
ID: 1049339702.Ph


1. NASA Goddard Space Flight Center,
Volume 4, Data for Advanced Space Communication and Tracking Systems
NASA Report NAS 5-9637, Prepared By Hughes Aircraft Co., Oct 1969

2. Curcio & Petty, Optical Absorption of Water,
Journal of the Optical Society of America (JOSA), V 41, pp. 302 -304

Reference 1. presents the spectral irradiance for sunlight and full
moon light on the surface of the earth under optimum conditions
(i. e. directly overhead in very clear air). Thus the lunar surface
faithfully reflects the solar spectrum without any wavelength selective
absorption. The intensity of the solar radiation on the earth's
surface under these conditions about 1000 watts per square meter
(0.1 watts per square centimeter) and the intensity of full moon light
is about one milliwatt per square meter (0.1 microwatt per square
centimeter). A milliwatt is one thousandth of a watt and a microwatt is
one millionth of a watt. Thus the power of full moon light on the earth's
surface is one million times less than full sunlight!

Because the spectrum of moonlight and sunlight is identical for both
sources of light, water will absorb and scatter penetrating light in
the same ratio as the intensity difference, one million to one.
Reference 2. presents data for very clear ocean water and very clear
fresh water. They have similar optical power absorption characteristics of
0.1 dB per meter of depth. This means that 2.28 % of the light power is
absorbed and scattered for each meter of depth and 97.72 % is transmitted
through each meter of depth. Of course these are very optimum conditions
and in murky water the absorption can be vastly different. However, for
organisms a few centimeters from the surface, water absorption of light
would not be an issue. The real issue is can bioluminescence occur with
optical power densities of moonlight as low as 0.1 microwatt per square
centimeter? You might ask that question to the Mad Science bioscience

Certainly our eyes can easily detect light at this very low level of
power density for we can see the full moon. However, under water, with
goggles, our eyes would function in sunlight at depths one million times
deeper than we can see the same objects with similar clarity in moon light.

Thank you for a very interesting question.

Best regards, Your Mad Scientist
Adrian Popa

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