Exploring Our Fluid Earth

Light Energy and the Electromagnetic Spectrum

Visible sunlight makes up about 40 percent of the total energy Earth receives from the sun. The rest of the energy Earth receives from the sun is not visible. About 50 percent is infrared energy, nine percent is ultraviolet (UV) energy, and one percent is X-rays or microwaves. Electromagnetic radiation is made up of electromagnetic waves that are defined by their wavelength and frequency. Of the entire electromagnetic spectrum, the human eye can view only a small portion of electromagnetic waves in the form of light.

Gamma (γ) rays, X-rays, and UV rays are types of electromagnetic waves that are high energy, with high frequencies and short wave lengths. These types of waves can be harmful to the human body if absorbed, and can generally penetrate more deeply because of their high energy. Ultraviolet (UV) radiation can destroy DNA and damage living organisms. Sunburnt skin is a painful example of the high-energy power of UV radiation. X-rays and gamma rays can pass through our bodies to make photos of bones and other internal organs. Large doses of these forms of electromagnetic radiation are very dangerous to living organisms.

Fig. 9.2. The electromagnetic spectrum with visible light highlighted

Image courtesy of Philip Ronan, Wikimedia Commons

Infrared (IR), microwave, and radio electromagnetic waves are low energy, with low frequencies and long wave lengths (Fig. 9.2). Lower energy waves are generally not harmful to the human body. Although our eyes cannot see infrared radiation, we feel the warmth from the heat it produces by jiggling whole molecules. Microwaves from a microwave oven can be used to reheat food from the inside out.

The electromagnetic spectrum describes the wide range of electromagnetic radiation forms (Fig. 9.2). Visible light, or light that can be seen by human eyes, accounts for only a small portion of the entire electromagnetic spectrum. Light from the sun or a light bulb appears white in color. However, white light is composed of several different wavelengths of light. White light shining through a prism reveals the different colors produced by different light wavelengths. (Fig. 9.3). Each light wavelength represents a different color in the visible light spectrum (Fig. 9.3).

Fig. 9.3. (B) White light passes through a prism and is refracted.

Image courtesy of D-Kuru, Wikimedia Commons

Fig. 9.3. (A) This animated diagram shows that different wavelengths of light are refracted through a prism. Click the thumbnail to see the animation.

Image courtesy of LucasVB, Wikimedia Commons

The visible light spectrum is a portion of the electromagnetic spectrum (Fig. 9.2). The visible light spectrum is composed of all of the colors of the rainbow. Each color is produced by a different wavelength of electromagnetic radiation. The color red has the longest wavelength within the visible light spectrum—approximately 650 nanometers (nm). at the opposite end of the visible light spectrum, the color violet has the shortest wavelength— about 400 nm. Wavelength, the distance between wave peaks, is a wave property linked with wave frequency. Waves with higher frequency (and thus, shorter wavelengths) generally have higher energy.

For a review of wavelength and wave frequency, see Wave and Wave Properties.

Electromagnetic radiation occurs in packets of energy called photons. A photon behaves like a wave and also like a particle. Because it is both a wave and a particle, describing the behavior of a photon is very complex. For convenience, scientists describe the amount of energy in a particular form of radiation in terms of its wavelength. Photons associated with different frequencies of light have different energies, and are utilized in different ways by ocean organisms.

Light Penetration in Water

Fig. 9.4. A shallow coral reef, an example of the euphotic zone

Image courtesy of Stefan Thiesan, Wikimedia Commons

Plants use sunlight as their primary source of energy through a process called photosynthesis. Photosynthetic organisms in the ocean such as algae and phytoplankton must live in well-lit surface waters called the euphotic zone (Fig. 9.4). The euphotic zone is the upper part of the ocean that receives bright and clear sunlight (Fig. 9.5). In clear tropical waters, the euphotic zone may extend to a depth of 80 meters (m). Sunlight does not penetrate as deeply near the poles, so in these areas the euphotic zone may be less than 10 m deep. Turbid, muddy waters may have a euphotic zone only a few centimeters in depth.

Fig. 9.5. Light penetration decreases with water depth in the ocean.

Image by Byron Inouye

The disphotic zone is the water layer beneath the euphotic zone (Fig. 9.5). In clear water it may extend as deep as 800 m. The dim blue light that penetrates this zone is not sufficient to sustain photosynthetic organisms. The photic zone is made up of the euphotic and disphotic zones. The aphotic zone is the water layer where there is no visible sunlight (Fig. 9.2). Most of the water in the ocean lies in the aphotic zone. Some lakes are also deep enough to have aphotic zones.

Colors in the Photic Zone

When sunlight strikes the ocean, some of it reflects off the surface back into the atmosphere. The amount of energy that penetrates the surface of the water depends on the angle at which the sunlight strikes the ocean. Near the equator, the sun’s rays strike the ocean almost perpendicular to the ocean’s surface. Near the poles, the sun’s rays strike the ocean at an angle, rather than directly. The direct angle of the sun’s rays to the surface of the water at the equator means that more energy penetrates the surface of the water at the equator than at the poles. Water absorbs almost all of the infrared energy from sunlight within 10 centimeters of the surface. In this very shallow layer light energy is converted to heat, which can raise the water temperature and cause some the water to evaporate. When winds and waves stir the surface of the ocean, heat mixes in to cooler water layers below.

For a review of how sunlight affects sea surface temperature, see Ocean Temperature Profiles.

Fig. 9.7. Visible colors of light penetrate differently into the ocean depths, as seen in this image depicting light penetration in Lake Superior. Longer wavelengths such as red are absorbed at a shallower depth than shorter wavelengths such as blue, which penetrates to a deeper depth.

Image courtesy of The University of Minnesota Sea Grant Program

Visible red light has slightly more energy than invisible infrared radiation and is more readily absorbed by water than other visible wavelengths (Fig. 9.7). This is why red fish appear nearly black at 20 m. Light with longer wavelengths is absorbed more quickly than that with shorter wavelengths. Because of this, the higher energy light with short wavelengths, such as blue, is able to penetrate more deeply. At 40 m, saltwater has absorbed nearly all the red visible light, yet blue light is still able to penetrate beyond these depths. At this depth, a scuba diver without a flashlight sees all underwater features only in shades of blue (Fig. 9.8 A). To see a full spectrum of colors, a diver must shine a white light directly on an object (Fig. 9.8 B).

Fig. 9.8. (A) A view of a mussel bed near New Zealand at 100 m depth, lit only by sunlight. Note the blue color tones.

Image courtesy of New Zealand-American Submarine Ring of Fire 2005 Exploration, National Oceanic and Atmospheric Administration (NOAA) Vents Program

Fig. 9.8. (B) A submarine spotlight illuminates a fish and colorful rocks off the coast of New Zealand.

Image courtesy of New Zealand-American Submarine Ring of Fire 2005 Exploration, National Oceanic and Atmospheric Administration (NOAA) Vents Program

Fig. 9.9. The intensity of sunlight decreases rapidly with depth.

Image by Byron Inouye

The depth of the water not only affects the colors of light that are noticeable underwater, it also affects the intensity, or amount of light. Within the first 10 m, water absorbs more than 50 percent of the visible light energy (Fig. 9.9). Even in clear tropical water only about 1 percent of visible light—mostly in the blue range—penetrates to 100 m. Light attenuation is the gradual decrease in light intensity as it travels through matter.