Electromagnetic Radiation


rainbow over Santa Barbara on Veteran's Day, 1997; photo by Sean Chamberlin

How many of us have stopped to stare at a rainbow and wished for a pot of gold? Nearly everyone finds some symbolic significance to a rainbow. Rainbows have inspired hopes and dreams since the dawn of humans. They are certainly one of the more stirring natural phenomena to occur on our planet.

While an oceanography class may not seem the place to be chasing rainbows, there is a lesson in rainbows that has relevance to our studies of the sea. Rainbows remind us that sunlight is composed of colors. What may seem like a pure stream of white light is actually a mixture of many different colors. And as we explore the effects of sunlight on our planet, the fate of sunlight in the oceans and, ultimately, the importance of different colors of light for photosynthesis in the ocean, it behooves us to know a little about this form of energy that we so often take for granted.

Physicists have two ways of looking at light: 1) as individual particles called photons; and 2) as a wave.  Both of these approaches are valid and both approaches yield fascinating insights into the properties of  light. And, as might be expected, both approaches generate arguments among physicists! This wave-particle duality is beyond the scope of this course but it is a mind-boggling tale that stretches every neuron of imagination in your brain. A couple great books for laypersons have been written on this topic, most notably, those by Richard Feynman, who coined the term quantum electrodynamics, or QED, to describe this field of study.

Let's look at the particle properties first. Light, or more properly, electromagnetic radiation, can be thought of as indivisible units called photons. You might think of photons as the "atoms" of light. These discrete packets of energy impinge on objects just like raindrops, only they are much faster and much smaller. Light through a vacuum travels at 300,000,000 meters per second. In other media, light travels more slowly; about 225,000,000 meters per second in the case of water. On a typical sunny day at noon, as many as 1,000,000,000,000,000,000,000 photons of light hit one square meter of pavement (or any other surface) every second. How fortunate that we cannot hear them! (The noise would be deafening and we would have to teach this class in the dark.)

Light also behaves like a wave, and, thus, every photon has a wavelength. It is these different wavelengths of light that appear as color to our eyes. We will learn more about waves later in the semester but here is one simple property of waves you need to know: wavelength. As shown below, wavelength is the distance between two successive crests (the peak) of a wave. Different colors of light and indeed, all the different types of electromagnetic radiation have different wavelengths.

Electromagnetic radiation is a kind of radiation that includes visible light (the light that we see), radio waves, gamma rays, X-rays and other forms of radiation in which the electric and magnetic fields vary simultaneously. For the purposes of our course, it's not important that we remember the electrical and magnetic part, but it is important that we recognize the types of electromagnetic radiation.

Electromagnetic radiation is divided into different types according to an electromagnetic spectrum. The part of the electromagnetic spectrum that we see is known as visible light. As shown in the figure below, visible light is just one small part of the electromagnetic spectrum, the part that has wavelengths between approximately 400 and 700 nanometers (10-9 meters, that's 0.000000001 meters). Cosmic rays and gamma rays occupy the short-wavelength end of the electromagnetic spectrum. They have wavelengths on the order of 10-14 to 10-10 meters. X-rays are between 10-10 and 10-8 and ultraviolet light is between 10-8 and 10-6 meters. Ultraviolet light is mostly filtered out by ozone in our atmosphere but as ozone concentrations in our atmosphere have fallen in recent years, more uv-radiation is reaching our planet's surface. UV-radiation is known to damage DNA, causing mutations and possibly cancer, so you can see why there is great concern for decreases in atmospheric ozone. Beyond visible light are longer wavelengths of light, like microwaves (between 1 millimeter and a meter long), radio waves (about the length of a yardstick and more) and electrical power waves, at wavelengths exceeding 10,000 meters.

The take-home message here is that the electromagnetic spectrum is characterized by radiation of different wavelengths. Each color that we see in a rainbow represents one particular wavelength of light, such as blue light which has a wavelength of approximately 480 nm. Shorter wavelengths, such as gamma rays, x-rays, and ultraviolet radiation, are less than 400 nm long, while long wavelengths, such as infrared radiation (heat), microwaves, radar, TV signals, and long-wave radio signals are greater than 700 nm and may be longer than a kilometer. FM radio and TV signals have a wavelength about the height of an average person. Maybe that's why we spend so much time paying attention to these signals.

Let's take a closer look at visible light. Visible light occupies only a small part of the electromagnetic spectrum.  We are able to see visible light because our eyes are only sensitive to visible light. That's why it's called visible light. But if we had eyes like hydrothermal vent shrimp, we would be able to see the heat patterns radiating from our fellow students and the food down at the cafeteria. Or if we had eyes like Jordie (spelling?) on Star Trek, who knows what we would be able to see!

Returning to rainbows, we notice that there is a pattern to the colors which occur. This color pattern is the same color pattern you would see if you were to transmit light through a prism (which is how Newton discovered that light was composed of colors). The long wavelengths of visible light are red and orange. The short wavelengths are blue and violet. In order of decreasing wavelength, here are the colors of the visible spectrum: Red, Orange, Yellow, Green, Blue, Violet. To help you, try this: ROY Gives Blue Violets. You can hardly miss. If you don't like this mnemonic, make one up of your own! (Try making one up for VBGYOR.)

Each of these colors are generally associated with specific wavelengths of light and you should familiarize yourself with them. Study the table below to get a feeling for the different wavelengths and their colors.

Color Wavelength (nanometers)
Red 650 - 700 nm
Orange 600 - 649 nm
Yellow 550 - 599 nm
Green 500 - 549 nm
Blue 450 - 499 nm
Violet 400 - 450 nm

Visible light having wavelengths greater than 700 nm is called far red or infrared, and with a proper pair of glasses, you can see light of this wavelength. Light at wavelengths below 400 nm is called ultraviolet light, a kind of light that has a tendency to zap molecules of DNA and thus create mutations that may lead to cancer. Ultraviolet light is what most good sunglasses try to remove to protect your eyes.

While we're on the subject of colors, do you know different materials have different colors? Do you know why trees are green and the ocean is blue? In the simplest explanation, it's because those are the colors that are reflected by those materials. All the other colors of light are absorbed. We'll get into more detail about the color of  the ocean when we examine the properties of the light in the ocean.

This information on the properties of light will help you better understand the world around you and help you understand the importance of light in ocean ecosystems. As you study these phenomena, you may want to refer back to this page.

Useful Links:

Exploring the Electromagnetic Spectrum
http://www.altair.org/index.html

The Electromagnetic Spectrum explained by NASA
http://imagine.gsfc.nasa.gov/docs/science/know_l2/emspectrum.html

The Amazing World of Electron and Photons
http://hyperion.advanced.org/16468/gather/english.htm