White is arguably the most common color in our daily lives. We see it everywhere – in the clouds, snow, milk, paper, walls of our homes and offices, and so much more. But what exactly makes the color white, white? What is happening on a molecular level that gives white objects and substances their bright, reflective appearance? In this article, we’ll explore the science behind what makes white color white.
What Is White Light?
To understand what makes the color white, first we need to talk about light. Visible light that humans can see exists on a spectrum of wavelengths. This light spectrum ranges from short wavelength violet light on one end, to long wavelength red light on the other end. White light contains all wavelengths of visible light. It is not one single wavelength, but a combination of all the colors in the visible light spectrum.
When all these wavelengths of light mix together and are emitted or reflected off an object, our eyes perceive this as the white color. So technically speaking, white is not really a color by itself, but a combination of all visible colors of light.
How Objects Appear White
For an object to appear white to our eyes, two things must happen:
1. The object must reflect or scatter all wavelengths of visible light equally.
2. The object must not absorb any of the wavelengths selectively.
Let’s break this down further:
Reflecting All Wavelengths of Light
For an object to look white, it must reflect back all the colors of light to our eyes relatively equally. If certain wavelengths are being absorbed while others are reflected, we will see the reflected colors rather than white.
For example, a leaf appears green because it is reflecting green light wavelengths while absorbing other colors like blue and red. In contrast, a white piece of paper reflects back all light wavelengths and does not selectively absorb any particular color. This makes it appear white to our vision.
Minimal Absorption of Light
The second criteria is that a white object must not absorb much light. The less light an object absorbs, the more it will reflect or scatter, and the whiter it will look.
Black objects, on the other hand, absorb almost all visible light wavelengths and reflect very little. That’s why they look black instead of white.
So in summary, when an object reflects back all colors of light while absorbing very little, it will appear white to our eyes.
What Makes Different White Substances White
Now let’s look at why some common white materials and objects appear white:
White Paint
White paint contains particles of titanium dioxide, zinc oxide, or lead oxide suspended in the paint medium. These particles help scatter and reflect all visible light wavelengths, causing the paint to appear bright white.
| White Paint Ingredient | Role in Creating White Appearance |
|---|---|
| Titanium dioxide particles | Reflect and scatter light waves |
| Zinc oxide particles | Reflect and scatter light waves |
| Lead oxide particles | Reflect and scatter light waves |
Paper
Paper is made of cellulose fibers from wood pulp or cotton. Air pockets between the fibers scatter light. Chemical whitening agents like bleach are also added to absorb less light and enhance the white appearance.
Clouds
Clouds are made of tiny water droplets or ice crystals that reflect and scatter all wavelengths of sunlight equally. This makes them appear white against the blue sky.
Snow and Ice
The ice crystals and air pockets in snow and ice reflect and refract light in all directions, causing them to appear white. Ice also contains tiny air bubbles that contribute to scattering light.
Milk
Milk gets its white color from casein protein molecules and fat globules that reflect and scatter light in all directions. The lactic acid in milk also helps scatter blue light, enhancing the white appearance.
Bones
Bones appear white due to calcium phosphate, a mineral that efficiently reflects all wavelengths of visible light. Bone is translucent, allowing light to pass through it and bounce off the calcium phosphate crystals back out to our eyes.
Teeth
Teeth look white because they contain a hard enamel coating made of hydroxyapatite crystals that effectively scatter all light wavelengths. Any stains or discoloration can reduce the light scattering and white appearance.
Salt
Table salt consists of white granules of sodium chloride. The chloride ions release electrons that interact with light waves, helping reflect and scatter the light to make salt look white.
Sugar
The sucrose molecules in refined sugar transmit and reflect all visible light, causing it to take on a bright white appearance.Molasses, on the other hand, contains impurities that selectively absorb light and give it a darker color.
True or Perceived White
When considering what makes something white, it’s also important to distinguish between “true” white and “off-white” or “perceived” white.
True white refers to objects and materials that reflect and scatter all visible wavelengths completely evenly. This results in a complete white appearance without any tint or hue. True white is rare in nature. Most natural whites are off-whites that we perceive as white.
Off-white or perceived white refers to objects and materials that don’t reflect all wavelengths perfectly evenly, but enough so that we still see them as white to our eyes. For example, paper, snow, milk, and salt are perceived whites that have slight tints of yellow, blue, or gray if you examine them closely. But our eyes and brain process them as white under normal lighting.
So when trying to understand what makes white white, keep in mind that most whites we encounter are not perfectly “true” white, but rather shades of white we perceive as white based on how they scatter light.
The Additive and Subtractive Nature of White
White can be created through both additive and subtractive color processes:
Additive White
With additive color mixing, white is created by combining all wavelengths of visible light. Computer screens and other light emitting devices produce white with the additive RGB color model. Red, green, and blue light combined together form white.
Subtractive White
Subtractive color mixing involves selecting which wavelengths to subtract or absorb from white light. With pigments and dyes, white is the natural color we get when no colors are absorbed from white light. For example with paints and inks, the more pigment added, the darker the color becomes. The absence of any pigment appears white because no colors are being selectively absorbed from the white light illuminating the surface.
So white arises through both the combination of all colors of light, and the absence of absorbing any color from light.
Whiteness and Perception
As we have discussed, whiteness arises from physics and chemistry – how light interacts with molecules, particles, and structures. But the perception of white is also highly psychological.
Our visual system and brain perceive slightly different shades of white as “white enough” based on factors like:
– Context
– Surrounding colors
– Lighting conditions
– Cultural definitions of white
– Individual differences in color vision
This means whiteness can be subjective. For example, an off-white wall appears even more white when surrounded by darker colors. And light cream may be considered white in one culture but not another.
So when considering what makes white white, it’s important to recognize both the objective physics and chemistry at play as well as our subjective psychological perceptions that ultimately create the experience of seeing the color white.
Conclusion
In summary, white is the presence of all visible wavelengths of light or the absence of absorbing any color from light. Objects appear white when they reflect and scatter all colors of light relatively equally without selectively absorbing any wavelengths.
Many natural and artificial white materials and objects exhibit these light reflecting and scattering properties through their chemical composition, structure, air pockets, or added pigments. Our eyes and visual system then translate this combination of wavelengths as the perception of white. But small variations in the precise diffraction of light lead to differences between “true” and “perceived” whites.
So while white may seem simple on the surface, the interaction of light physics, material chemistry, and visual neuroscience is complex. The next time you see a field of snow, clouds in the sky, or white paint on a wall, you’ll have a deeper appreciation for the science behind what makes white color white.
