5.11: Transmission of Light and Other Electromagnetic Radiation

Light is a form of electromagnetic radiation emitted by the sun and stars. However, visible light occupies only a small segment of the very wide electromagnetic radiation spectrum (Fig. 5-17). Ultraviolet light, X-rays, and gamma rays have progressively shorter wavelengths than visible light, whereas infrared light, microwaves, and radio waves have progressively longer wavelengths.

Figure 5-17. The electromagnetic radiation spectrum.

Absorption

Water absorbs electromagnetic radiation, but the depth of water penetrated varies with the intensity of the radiation and with wavelength. At most wavelengths, absorption is so effective that, even at very high intensity, the radiation can penetrate only a few centimeters or meters of water. Absorption is less effective within the narrow range of wavelengths of visible light and for very long-wavelength radio waves. However, even at the wavelengths of lowest absorption, the most intense radiation (e.g., lasers) cannot penetrate more than a few tens or hundreds of meters of water. For this reason, oceanographers cannot use electromagnetic radiation, such as radar, radio waves, or laser light, to “see” through the oceans in the same way that radar can see through clouds to the Earth’s surface or even the surface of Venus.

Because radio waves and other forms of electromagnetic radiation do not penetrate far through water, they also cannot be used for underwater communication. For example, long-range communication with submarines is a considerable problem. Very low-frequency radio waves can penetrate a few tens of meters into the ocean and are used to communicate with submarines near the surface. Because intense visible light can penetrate up to several hundred meters below the ocean surface, satellite-mounted lasers also are used to communicate with submarines at shallow depths. The inability of electromagnetic radiation, such as radar, to penetrate ocean water is the principal reason why submarines are among the best places to hide strategic missiles. Detection of submarines relies primarily on sound waves, which are transmitted easily through ocean water.

The greater transmissibility of visible light through water (compared to other electromagnetic wavelengths) is critically important to life in the oceans (Chaps. 12, 13). Like life on the land, most non-microbial life in the oceans depends on algae and bacteria that need light to grow by phototrophy (CC14). Since light energy decreases rapidly with depth in the oceans, phototrophic activity also decreases with depth until the light energy is insufficient to sustain growth and reproduction. To define this depth, ocean scientists use the depth at which the light intensity is reduced to 1% of that at the ocean surface. Water above this level is referred to as the photic zone and water below as the aphotic zone. 

Some wavelengths of visible light are better absorbed by water than others (Fig. 5-18a). Wavelengths at the red end of the visible spectrum are the most rapidly absorbed, and violet wavelengths also are absorbed relatively quickly; blue and green light penetrate farthest. Outside the visible light spectrum, infrared radiation (wavelengths longer than red light) and ultraviolet radiation (wavelengths shorter than violet light) are strongly absorbed by water.

Figure 5-18. Light penetration and absorption in ocean waters. (a) Absorption of electromagnetic energy by ocean water. Note that energy in the infrared (about 700–1000 nm) and ultraviolet (about 300–400 nm) wavelengths is absorbed very rapidly compared to energy in the visible light wavelengths (400–680 nm). Also, energy at longer wavelengths of visible light (red/orange) is absorbed more quickly than energy at shorter (blue/green) wavelengths. The remaining parts of the figure show the depth at which light of visible wavelengths is reduced to less than 1% of the intensity at the surface, the demarcation between the photic and aphotic zones in (b) clear open-ocean water, (c) coastal-ocean water, and (d) estuarine water. Note that the photic zone depth is much reduced in coastal and estuarine waters. Below the photic zone, light is insufficient for phytoplankton to produce enough organic matter by photosynthesis to survive. Note also that the wavelength range of light reaching the lower part of the photic zone moves from the blue/green part of the spectrum toward the red end of the spectrum in coastal and estuarine waters. This is the reason why open-ocean waters usually appear blue while coastal and estuarine waters do not.

As light penetrates the ocean, it is absorbed not only by water molecules, but also by suspended particles. Accordingly, light penetrates deeper in clear ocean waters than in high-turbidity coastal waters with high suspended sediment loads or high plankton concentrations (Fig. 5-18b-d).

Scattering and Reflection

We are all familiar with the idea that light is reflected by smooth “shiny” surfaces such as the silvered surface of a mirror. Light is also reflected when it encounters a rough surface. However, individual light rays meet different parts of the rough surface at different angles, and as a result, they are reflected in different directions. When this occurs, the light is said to be scattered.

As light penetrates ocean water, scattering occurs when some of the photons of light bounce off water molecules, molecules of dissolved substances, or suspended particles. The light scatters in all directions, but some is directed upward and is said to be backscattered. In very clear ocean waters with few suspended particles or plankton, most of the red and yellow light is absorbed rather than scattered (Fig. 5-18b-d). Light of blue wavelengths has a greater probability of being backscattered because blue light travels through a greater length of the water column. Therefore, most of the backscattered light in clear ocean waters is of blue wavelengths, and it is this backscattered light that gives the ocean its deep blue color.

In waters with more suspended particles or plankton, there is a higher probability of backscattering of all visible wavelengths. Hence, in more turbid waters, the backscattered light comprises a much wider range of wavelengths, and the water appears not blue, but green or brownish. The specific color is determined by the nature of the suspended particles. All particles absorb light more effectively at some wavelengths, and their color is determined by which wavelengths of the light they do not absorb. Light of these nonabsorbed wavelengths is scattered and, therefore, constitutes the wavelengths we see when looking at the particles. Most particles in the oceans are either green, such as many phytoplankton cells, or brown, such as sand and other mineral particles. Ocean waters with large phytoplankton populations (Chap. 14) tend to appear greenish, whereas coastal waters with large loads of suspended sediment (mineral grains) appear muddy brown. Therefore, color variations of the ocean surface indicate the quantity of particulate material and phytoplankton in the near-surface water. Ocean surface color measured by sophisticated sensors aboard satellites (Fig. 5-19) is used to investigate distributions of the living and nonliving particles in the oceans (Chaps. 6, 12).

Figure 5-19. This image of the Northwest Atlantic Ocean shows data obtained from a satellite-mounted instrument called the Ocean Color Instrument, which measures the intensity of light reflected or backscattered by the Earth’s surface. The data are from a narrow wavelength band, in which the intensity of the light received by the scanner from the ocean surface increases with increasing concentrations of chlorophyll-containing phytoplankton in the photic zone. Intensities of light in this wavelength band are depicted by the false colors in this image. Red colors show the areas of highest chlorophyll and phytoplankton concentration; and progressively lower concentrations are depicted by the orange, yellow, green, and blue shades. Note that the highest concentrations of chlorophyll are found in the coastal region and that the image reveals the complex patterns associated with different water masses.

The underwater world seen in movies or video is a blaze of colored creatures. However, scuba divers see such colors only in very shallow water. Just a few meters below the surface, the red and yellow wavelengths of natural sunlight are absorbed, so bright red fishes appear black and most objects appear blue or bluish green. The true colors are revealed only when lights with a full spectrum of visible wavelengths are shone on the marine life. For this reason, underwater photographers or videographers must carry powerful lights.

By noting changes in the visibility and lighting of the water, scuba divers can observe scattering and absorption of light by particles. In turbid water with more particles, daylight does not penetrate very far. In addition, at shallow levels to which some natural light penetrates, horizontal visibility is reduced. Just as daylight is blocked from penetrating vertically into the water by scattering and absorption, light traveling from an object horizontal to the diver is absorbed and scattered. If the absorption and scattering are substantial, the light from distant objects does not reach the diver and the objects cannot be seen. When light is scattered equally in all directions, a scuba diver may not be able to visually distinguish between up and down.

Refraction

The speed of light in seawater is less than that in air. As light waves pass from air into water or from water into air, they are refracted (their direction is changed because of the change in speed) (Fig. 5-20a). An easy way to see refraction is to sight down the length of a ruler and dip the end of the ruler into a tumbler of water (Fig. 5-20b). The tip of the ruler appears to be bent upward as it passes into the water. Refraction causes fishes in an aquarium to appear bigger and closer than they really are. Similarly, fishes and other objects seen through a scuba mask appear larger and closer than they are.

Figure 5-20. (a) Light rays are bent, or refracted, when they pass between air and water. (b) Inserting a ruler or stick through a water surface at an angle illustrates the distortion in the way refraction makes objects appear. The ruler looks as if it is bent at the point it enters the water.

Refraction between air and water increases as salinity increases. Therefore, measurements of refraction are often used to determine approximate salinity.