Resource: Primary Colors of Light and Pigments

The inner surfaces of your eyes contain photoreceptors—specialized cells that are sensitive to light and relay messages to your brain. There are two types of photoreceptors: cones (which are sensitive to color) and rods (which are more sensitive to intensity). You are able to "see" an object when light from the object enters your eyes and strikes these photoreceptors.

Some objects are luminous and give off their own light; all other objects can only be seen if they reflect light into your eyes. However, humans can only see visible light, a narrow band of the electromagnetic spectrum (which also includes non-visible radio waves, infrared light, ultraviolet light, X-rays, and gamma rays). In terms of wavelengths, visible light ranges from about 400 nm to 700 nm.

Different wavelengths of light are perceived as different colors. For example, light with a wavelength of about 400 nm is seen as violet, and light with a wavelength of about 700 nm is seen as red. However, it is not typical to see light of a single wavelength. You are able to perceive all colors because there are three sets of cones in your eyes—one set that is most sensitive to red light, another that is most sensitive to green light, and a third that is most sensitive to blue light.

Red, green, and blue are the primary colors of light—they can be combined in different proportions to make all other colors. For example, red light and green light added together are seen as yellow light. This additive color system is used by light sources, such as televisions and computer monitors, to create a wide range of colors. When different proportions of red, green, and blue light enter your eye, your brain is able to interpret the different combinations as different colors.

However, there is another set of primary colors with which you may be more familiar. The primary colors of pigment (also known as subtractive primaries) are used when producing colors from reflected light; for example, when mixing paint or using a color printer. The primary colors of pigment are magenta, yellow, and cyan (commonly simplified as red, yellow, and blue).

Pigments are chemicals that absorb selective wavelengths—they prevent certain wavelengths of light from being transmitted or reflected. Because paints contain pigments, when white light (which is composed of red, green, and blue light) shines on colored paint, only some of the wavelengths of light are reflected. For example, cyan paint absorbs red light but reflects blue and green light; yellow paint absorbs blue light but reflects red and green light. If cyan paint is mixed with yellow paint, you see green paint because both red and blue light are absorbed and only green light is reflected.

To learn more about light, check out How Light Travels and Light and the Law of Reflection.

To learn more about the colors of light, check out Light and Color.

To learn more about the electromagnetic spectrum, check out The Electromagnetic Spectrum: FRONTLINE and The Electromagnetic Spectrum: NASA.

To learn more about how the eye works, check out Cow's Eye Dissection.

• What is a photon? Are all photons alike?
• When white light goes into a blue filter, blue light comes out. How did the blue get into the light?
• When red light passes through a green filter, no light gets through. What happens to it?
• When red light is projected onto a white surface, red light is reflected. Blue light will similarly be reflected as blue. When both are projected onto a white surface, we see neither red nor blue. Is the red light still there? Is the blue light still there? What happened?
• Explain why a sweater looks red. Start with sunlight or white light from a lamp striking the sweater.