Light and Other Electromagnetic Radiation

What is electromagnetic radiation? Electromagnetic radiation includes light and other "rays" from the sun, lights, X-rays, microwave ovens, radio station antennas, and other sources. All electromagnetic radiation travels in a wave form, like shown below.

Waves and Wavelengths. The wave-like movement, or "cycling", up and down from the peak to the trough (pronounced "troff") and returning back to the peak is one "wave" or one "cycle." The length of one "wave" is called the "wavelength." One wavelength is the straight-line distance between two adjacent peaks (shown below). 

Different types of electromagnetic radiation (including light) have different wavelengths.

Frequency: A second very important characteristic of electromagnetic (EM) radiation is the frequency of the wave. The frequency of a particular type of EM radiation is the number of wavelengths the radiation travels in one second.

For example, in the example below, the (extremely long-wavelength) radiation has traveled 6 wavelengths in one second. So, its frequency would be 6 Hz.

The frequency of radiation is measured in Hertz (Hz). For example, the frequency of red visible light is about 4*10¹⁴ Hz. This means that in one second, infrared radiation cycles through 4*10¹⁴ (or 400,000,000,000,000) wavelengths!

The frequency of radiation is important because it is related to the amount of energy of that particular kind of radiation: the higher its frequency, the more energy it has.

Different kinds of EM radiation. Since the sun is a major source of light and other radiation here on Earth, we'll talk about the sun's radiation. The sun's rays includes radiation that vary in wavelength, frequency, and energy. (See the picture below.)

The sun sends out lower-energy radiation such as radio waves, microwaves, and infrared (pronounced: in-fra-RED) radiation. ("Infrared" is from the Latin: "Infra" meaning "below" and "red" as in the color red. :-).) Infrared radiation is lower in energy than red visible light.

The picture below shows these and other types of electromagnetic radiation, ordered from highest to lowest wavelength — or lowest to highest frequency/energy.

The middle-energy radiation from the sun that we are able to see is what we call visible light (or simply "light"). And the sun emits higher-energy radiation called ultraviolet radiation. (Ultraviolet is from the Latin: "Ultra" meaning "beyond" and "violet" as in the color violet.) So, ultraviolet radiation is higher in energy than violet visible light.

The sun also emits even higher-energy radiation: X-rays and Gamma rays. The very high-energy gamma rays are what made David Banner the Incredible Hulk in comic books. But in reality, exposure to gamma radiation will cause you lots of health problems! Lucky for us, the Earth's atmosphere filters out this high-energy radiation from the Sun.

As we mentioned before, different types of radiation have different wavelengths and frequencies. Radio waves have longer wavelengths (and lower frequencies) than microwaves; microwaves have longer wavelengths (and lower frequencies) than infrared radiation; etc.

Wavelengths and frequencies of visible light. Just like with the different types of radiation (ultraviolet, infrared, etc.), the wavelengths of visible light vary as well. As shown below, light, since it's a form of EM radiation, travels in a wave-like form. One wavelength of each color of light is highlighted in the picture below. For example, violet has a wavelength of 400 nm (nanometers). One nanometer is equal to 0.000000001 meters, or one-billionth of a meter (10^-9 meters).

The chart below shows differences in the wavelengths and frequencies of various colors of light in the visible spectrum (of course, the wavelengths of actual light are much, much, much, much smaller!). But this is to give you some idea about how the wavelengths of different colors of light compare to each other.

Just how small is 405 nanometers (the wavelength of violet light)?

405 nm is equal to 405 × 10^-9 meters. This is equal to about 4 × 10^-7 meters. Looking at the electromagnetic spectrum picture above, this is about the size of a one-celled animal, like an amoeba.

You can imagine this length by taking a meter stick and looking at one centimeter, which is about half the diameter of a penny (see below). One centimeter is one-hundredth of a meter (or 10^-2 meters). Then divide the centimeter by 10, which is one millimeter, or one-thousandth of a meter (10^-3 meters). In the picture below, one millimeter is the distance between the shortest lines on the ruler or 1/10th of a centimeter.

Then divide the millimeters into about 1,000 more parts. That distance would be too small for your eyes to detect. But it will still be a bit longer than the wavelength of violet light! In fact, that will be longer than even the wavelength of red light, which has the longest wavelength!

Speed of light. You might think that radiation that has longer wavelengths might travel faster. (Do you?)

But actually, all forms of radiation travel at the same speed (about 3x10⁸ meters per second, or about 300,000,000 m/s), regardless of wavelength. This is equal to 671,000,000 miles per hour!

So, the speed of violet light is the same as the speed of red light, which is also the same as the speed of ultraviolet radiation, gamma rays, and (at the other extreme lower end of wavelengths), radio waves. 

Because all light travels at the same speed, it takes less time for shorter-wavelength radiation to move a distance of one wavelength and it takes more time for longer-wavelength radiation (for example, radio waves) to move a distance of one wavelength. Scientists call the time it takes for light to travel a distance of one wavelength (or one cycle) its period.

Period: The period of a light wave is the time it takes the light to travel one cycle (or one wavelength). The period is inversely related to the frequency: p = 1/f. This also means that the frequency is inversely related to the period: f = 1/p. 

Because all light travels at the same speed, light with longer wavelengths (like infrared and red light) has lower frequencies. And light with shorter wavelengths (like violet visible light and ultraviolet radiation) has higher frequencies.

((A bit more Advanced due to Math with exponents. If you need help on math with exponents, click here.))

Speed of light. We can figure out the speed of light using the basic definition of speed: the distance an object moves divided by the time it takes the object to move that distance. 

Analogy: To make this section a little easier to understand, let's think about someone walking at a constant speed. And let's say that each step they take is equal to one meter. Also, it takes them one second to take each step. So, this person moves one meter in one second. And their speed is simply one meter per second. It was calculated as:

Speed = distance (per step) / time (per step).

For light, speed can be calculated as the distance light travels divided by the time light takes to travel that distance. 

Because one wavelength is the basic unit of distance for light (like one step was in the analogy above), we can use that as the distance light travels. The light will travel that distance (one wavelength) in one period.

So, we write the speed of light as:

Speed of light = One wavelength (meters)/period (seconds)

The period is the time light takes to travel one cycle (one wave). The inverse of the period (1/period) is the number of cycles (or waves) that light travels in one second. 

The number of cycles that light travels in one second is called its "frequency." So, we can replace the period in the equation above with the frequency.

Speed of light = One wavelength (meters) x frequency (cycles per second)

Because the speed of light is the same for all types of light, light that has longer wavelengths (such as red light) must have a lower frequency than light with shorter wavelengths (such as violet light).

Calculating the frequency of light.  We know (from experiments) that light travels at about 3 x 10⁸ meters/second. We also know the wavelengths of different types of light. So, we can calculate the frequencies for different types of lights. Let's look at red light.

3 x 10⁸ m/s = 6.5 x 10^-7 m ×  frequency

Dividing both sides of the equation by 4 x 10^-7 m:

.46 x 10¹⁵ cycles/second = 4.6 x 10¹⁴ cycles/second = frequency

This means that in just one second, red light cycles 4.6 x 10¹⁴ times! In other words, in just one second, red light travels through 4.6 x 10¹⁴ waves! Pretty fast!!

((End of Advanced section.))

Experiment! You can calculate the speed of microwaves, one type of electromagnetic radiation, by heating stale peeps in your microwave!

The keys things to remember for this experiment are: (a) make sure to use stale peeps, and (b) make sure the peeps do not rotate in the microwave when they're heated.