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12. Light Penetration in the Ocean
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The Briny Deep

1. Introduction
2.  Salt in Seawater?
3. Ions of Salt
4. Source of the Salt?
5. Salinity Variability
6.Salinity & Precipitation
7. Salinity Summary
8. Calculating Salinity
9. Ocean Temperature
10. Solar Radiation
11. Properties
12. Light Penetration
13. Temperature
14. Thermocline
15. Density
16. Layering
Used with Permission of Matthias Tomczak ęCopyright 1997
  • The colored curves on the graph above show the intensity of light (solar radiation) at different depths in the ocean plotted against wavelength of light at each depth. The colors indicate light intensity at four water depths:
  • 0 meters

    0.01 meter = slightly less than a half inch

    0.5 meters=20 inches

    100 meters=330 feet


Light absorption in the sea reduces the amount of visible light rapidly with depth.

  • Also note that absorption is greatest for the long wavelengths of light (measured in millionths of a meter or in microns " µ ") and somewhat less so for shorter wavelengths of light.

The colors that you can see beneath the sea depend on the wavelength of light available to illuminate an object.

  • A common observation is that a white plate will appear light blue underwater, because the long wavelengths of light, which include the red colors, have been absorbed in the surface water and only shorter wavelengths of light associated with blue colors remain to illuminate underwater objects.
Graph showing light wavelentgh and penetration into water
Used with Permission of Matthias Tomczak ©Copyright 1997

Also, the intensity of this light decreases rapidly with water depth, for example,

  • only 73% of the surface light reaches a depth of 1 centimeter (less than a half inch)
  • only 44.5% of the surface light reaches a depth of 1 meter (3.3 feet)
  • 22.2% of the surface light reaches a depth of 10 meters (33 feet)
  • 0.53% of the surface light reaches a depth of 100 meters (330 feet)
  • 0.0062% of the surface light reaches a depth of 200 meters

Bottom line  -- most of the light is absorbed or scattered within the top few meters of the ocean.

Indeed by 100 to 200 meters deep, virtually all of the solar radiation has been absorbed......remember that the abyssal plains, which cover the vast majority of the ocean basin, are between 4000 and 5500 meters deep, consequently the vast majority of the ocean is dark and cold!!!.

The minimum energy supply necessary to maintain photosynthesis is 0.003 cal per cm2. Under most conditions this level of energy, and therefore photosynthesis, is only available to the upper 200 meters of the sea. The surface layer of water where photosynthesis can take place is called the euphotic zone, or just the photic zone.

ęCopyright 1999
March 13, 1999

Send to Don Reed

Department of Geology
San Jose State University


#6 Why doesn't light penetrate far into the ocean?

a) Because most of the light reflects from the surface.
b) Because most of the light is absorbed at the surface.
c) Both a and b.
d) None of the above.