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The Nature of Light: Origin, Spectrum & Color Frequency

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  • 0:01 Perception of Color
  • 0:32 Visible Light
  • 1:58 Colors in the Spectrum
  • 5:54 Naming the Colors
  • 6:42 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

This lesson introduces the basics of visible light and color. Learn how the visible light spectrum is divided into the six color ranges. We'll also discuss why different people name colors in different ways.

Perception of Color

Have you ever wondered if other people see colors the same way you do? If you say that your shirt is blue, and your friends say that your shirt is blue, are they really seeing the same blue that you are? And what about different varieties of blue? Some blues have more of a greenish tinge, while others look more violet. One of your friends might call your shirt teal, while another might call it aquamarine. How do we perceive different colors and distinguish between them? To answer these questions, we'll have to learn more about the visible light spectrum.

Visible Light on the EM Spectrum

The visible light spectrum is a very small part of our giant electromagnetic spectrum. The EM spectrum consists of seven different regions, including x-rays, infrared, and ultraviolet light. The electromagnetic waves that we can see are confined to this small area, called the visible light region. This part of the spectrum is the range of light frequencies to which the human eye is most sensitive. Visible light shares many characteristics with the other EM waves, because it, too, is a type of electromagnetic radiation.

All electromagnetic waves originate from the vibration of charged particles. These charged particles come from a variety of sources, like the sun, other stars, and warm objects like light bulbs and animals. Any source of EM radiation that we can see - like the light from the sun - is considered visible light. So, other sources of visible light would be flames, red-hot metal, and the glowing screen on your cell phone. Our ability to see everyday objects is due to the reflection of light from the surfaces of those objects. For example, the reason you can see your car in your driveway is because the sunlight is reflecting off of it. Your car may be getting very warm in the sun, and it may be giving off infrared radiation because of that warmth. But that's not a form of radiation you can see. You can only see the waves in the visible light region.

Colors in the Spectrum

Recall that the regions in the EM spectrum are distinguished by their ranges in wavelength as well as frequency. EM waves with a large wavelength have a low frequency, and waves with a small wavelength have a high frequency. Visible light spans from about 760 to 380 nanometers in wavelength, which is the same as about 430 terahertz to about 750 terahertz. The low-frequency end corresponds to light that we perceive as the color red. The high-frequency end is related to light that we see as violet. EM waves that are just below the red spectrum are called infrared. These are the waves that have a frequency of less than 300 THz, or more than 1000 nm. We can't see these waves, but we can detect them as heat. At the other end, above the limit for violet light, is the ultraviolet spectrum. Ultraviolet waves have a frequency of more than 1000 THz, or a wavelength of less than 300 nm. We can't see ultraviolet light, either, but we can certainly tell when our skin has been burned by UV exposure.

So, we know the lower frequency limit for red. We know the upper frequency limit for violet. But what happens in between those limits? Where do red and violet transition to the other colors in our visible light spectrum? Red light spans up to about 480 THz, or 635 nm, at which point it transitions into orange. Orange light spans up to 510 THz, or 590 nm. And yellow light spans up to 540 THz, or 560 nm. Do you see how the wavelengths are dropping lower as we increase in value for frequency? That's because for EM waves, wavelength is inversely proportional to frequency. You've seen this on the larger scale of the entire EM spectrum, but it holds true for the tinier bands of colored light on the visible light spectrum.

Green light comes after yellow, and it spans up to 610 THz, or 490 nm. Then it transitions to blue until 670 THz, or 450 nm. Violet comes last, ending around 750 THz or 400 nm; after which it switches to the ultraviolet light range. It's important to note that the colors we're describing here would be produced by a single frequency of light. For example, light with a frequency of 650 THz would look blue. We would call this blue a pure spectral color, because it is only produced by a single frequency, or a single wavelength, of visible light. Each different frequency on the blue color range would produce a slightly different shade of blue, so that at some point the color of light would look green or violet.

The point of transition is something that scientists approximate based on how people perceive the frequencies. So you know those frequency ranges I gave you? Well, they're not exactly definite rules. The pure spectral colors form a continuous spectrum, just like the regions of EM waves form a continuous EM spectrum. There's no official cut-off mark for any of the colors. So while blue is typically classified as lying between 610 and 670 THz, you'll find some people that identify blue at higher frequencies.

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