Roses are red, Violets are blue,

And you probably think

That the sky is blue too.

Color, however, exists only in the mind: Color is our experience that maps onto the physical luminance properties of visible light and visible-light reflectance properties of objects. Psychologists call this color perception, to recognize that color is more a property of us than of the physical world. Experience is part of the reason bananas look yellow even in dim or unusual lighting conditions – we expect them to be yellow, so we see them as yellow even when yellow is hard to distinguish from other colors. [See this post for a definition of this phenomenon, color constancy, and others. Check out Emma’s post on color-grapheme synesthesia for some really unique color experiences, or see this post on the universal part of the experience of red.]

The rest of this post will focus on the physics of light, and how that relates to color perception. Visible light is a narrow range of wavelengths of light that reflect off of most objects (black is the absence of reflected light). Ultraviolet a.k.a. UV light (shorter wavelengths) passes through objects and infrared (longer wavelengths) reflects off fewer objects, so the light we use to see is the most useful for our purposes of navigating and interacting with our environment.

Our names for light wavelengths outside of visible light indicate the colors we perceive at shorter (violet) and longer (red) visible wavelengths when only a single wavelength of light is present. Rainbows with their purple inside curves and red outside curves illustrate the color spectrum precisely because the sunlight passing through raindrops is filtered so that individual wavelengths are separated out in order.

But we see sunlight as mostly white, not rainbow, most of the time. Why? We generally are exposed to mixtures of multiple wavelengths of light. White light is an example: it includes wavelengths across the spectrum of visible light, so it is also known as broadband light. In this way, light is the opposite of paint: the more colors you add, the closer it gets to white.

So far, we have addressed colors of light, yet we see objects as having color as well. This is due to reflected light bouncing off objects and into our eyes. Objects do not perfectly reflect back all the light that hits them – or we would see everything as the same color or even see nothing at all. Instead, they absorb some wavelengths of light. That is why mixing different colors of paints yields darker colors: each color added absorbs another set of wavelengths of light, until no light is reflected (black).

As alluded to above, color is not as simple as wavelengths of light and reflectance. We care less about specific reflectance patterns under a specific lighting condition than we do about the material of objects: color constancy is the name for the phenomenon that my blue dress looks blue in both the store’s artificial light and outside in sunlight. We don’t think that wearing the dress outside suddenly makes it something different, so we see the same color even though the exact light reflectance does differ.

Though light physics explains a portion of color perception, color constancy reminds us that color is a convenient mental interpretation of the physical world – not a physical fact.

[Interested in more vision and color research? Find out about how language and color perception interact here, about how color is involved in food marketing here, about other visual constancies and phenomena here, and about how 3D movies work here.]