By Robert M. Hazen, Ph.D., George Mason University
In the early days of the discovery of electromagnetism, the only form of it known to scientists was visible light. And visible light has a very narrow range of wavelengths, from about fifteen- to about thirty-millionths of an inch. Where were all the different kinds of waves? These were predicted by scientific theory, but nobody had seen them before. How can this spectrum be sensed?
Understanding Waves
Imagine yourself in a rowboat on the ocean. The surface of the ocean can display many, many different wavelengths. You can have microscopic ripples barely visible. You can have globe-spanning tides, waves that are essentially thousands of miles long. And you can have all sorts of waves in between.
Now the rowboat responds only to waves that are fairly close to the length of the rowboat. If you have waves that are 10 or 20 or 30 feet in wavelength, the rowboat’s going to pitch and turn; you’re going to feel those quite dramatically. And that’s what you’re going to be able to sense. But unless you are looking very closely at the surface of the ocean, you’d never see the tiny ripples. You wouldn’t feel those. Nor would you feel the tides.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Beyond Visible Light
And this is very much like the situation with humans and the electromagnetic spectrum. Humans have only their eyes as detectors, and eyes can only see the visible range. So we never suspected the existence of many other wavelengths.
And it was the discovery and the use of these different waves that have really transformed technology—particularly during the period 1880 to 1930—transformed society in many, many different ways because of the discovery of other forms of electromagnetic radiation.
The only way for us to know about electromagnetic radiation is for it to interact with matter. It turns out that there are only three basic ways electromagnetic radiation interacts with matter: they’re called transmission, absorption, and scattering. And all three you experience all the time in your daily life.
Learn more about the nature of energy.
The Process of Transmission
Transmission is when an electromagnetic wave passes right through an object. If an object is transparent, that’s because the light passes through it. So window glass is transparent to visible light. Now, transmitted light can experience two important and related effects. First of all, the waves are often slightly bent.
This is the phenomenon known as refraction. Refraction of light by a glass is the basis of all sorts of eyeglasses and optical devices, and you can see this phenomenon, of course, through a magnifying lens that you bend light, and therefore change the appearance of the object behind that lens. We also use telescopes and microscopes, very important sort of devices.
Light Slows Down
In addition, whenever light is transmitted through material, at least anything other than a vacuum, it slows down. This is a remarkable phenomenon. The amount of that slowdown is directly related to the amount of refraction, the amount of bending that you see with the lens.
A diamond is one of the highest indices of refraction. Indeed, light slows down to a paltry 80,000 miles per second—that’s less than half of its speed in a vacuum. So as soon as light hits a diamond, it just puts on the brakes and creeps on through at 80,000 miles per second.
Absorption: The Interaction of Light with Matter
The second way light interacts with matter is called absorption. Electromagnetic waves are a form of energy. That energy can be transferred from one object to another, and it can be absorbed by materials, and that absorption is one of the processes that happens. When this happens, the light is often converted to heat energy.
You’ve experienced this phenomenon if you’ve ever walked on the blacktop in the summer. Blacktop soaks up the sun’s energy, it gets very, very hot, and if you put your feet on it, you really can burn yourself. That’s why you often see people walking along the white lines of a parking lot in the middle of summer if they have bare feet.
So any black material is basically absorbing the light. It’s going to be much warmer in the wintertime, but it’s also going to be much hotter in the summertime to wear black materials.
Learn more about the laws of thermodynamics.
The Phenomenon of Scattering
And then we come to the third kind of interaction; that’s called scattering. That’s when electromagnetic waves bounce off a material and scatter off the surface, and this can happen in several different ways. You have diffused scattering. Diffused scattering is just what happens when light goes off a white surface, and light rays come in and they bounce off in all directions equally. The appearance of the surface is just white.
You also have the phenomenon of reflection. And in reflection, you have a much more uniform kind of scattering. You find, in this case, the light bounces off quite regularly—a reflection off a mirror. And then you have a third kind of scattering called diffraction.
Diffraction is a very special kind of scattering in which light waves are split up. You may have seen the kind of refraction effects you get off a CD when you see that rainbow of colors. Light is being broken up into its component colors by this process. Iridescence is the process of diffraction.
Conservation of Energy
The conservation of energy requires that the energy of electromagnetic energy of radiation hitting a material has to be equal to the energy that is transmitted, absorbed, and scattered. So there’s the conservation of energy here always. And often you’ll see all three of these effects occurring simultaneously.
For example, if you shine a light through colored water, you have some transmission, you have some scattering, and you have some absorption. All three processes are occurring simultaneously.
For humans, it is the visible spectrum that interacts with our eyes. Light reflected and scattered by the world around us interacts with our eyes, and that is how we see things around us.
Common Questions about Transmission, Absorption, and Scattering
The three ways in which electromagnetic waves interact with matter are transmission, absorption, and scattering.
Transmission is when an electromagnetic wave passes through an object. If an object is transparent, that’s because the light passes through it.
Electromagnetic waves are a form of energy. That energy can be transferred from one object to another, and it can be absorbed by materials. This is called absorption.
Scattering is when electromagnetic waves bounce off a material and scatter off the surface, and this can happen in several different ways.
