White light is made up of all the colors of the visible light spectrum. When white light shines through a prism, it separates into the seven colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet. This occurs because each wavelength (color) of light bends slightly differently when it passes through the prism, causing the colors to become visible.
The Visible Light Spectrum
The visible light spectrum is the range of wavelengths or frequencies of electromagnetic radiation that can be detected by the human eye. The visible spectrum runs from about 400 nanometers to 700 nanometers.
| Color | Wavelength Range (nm) |
|---|---|
| Violet | 380-450 |
| Blue | 450-495 |
| Green | 495-570 |
| Yellow | 570-590 |
| Orange | 590-620 |
| Red | 620-750 |
Each color has a different wavelength, with violet having the shortest wavelength and red having the longest. When all these wavelengths of visible light are combined together, they make white light.
Light as a Wave
Light can be described as both a wave and a particle. As a wave, light has specific properties, including wavelength, frequency, and amplitude.
Wavelength is the distance between successive wave peaks, while frequency refers to the number of wave cycles that pass a fixed point per unit of time. Amplitude is the height of the wave from peak to trough.
The wavelength determines the color of visible light. Shorter wavelengths are violet and blue light, while longer wavelengths are red and orange. Wavelength and frequency are inversely related – as wavelength decreases, frequency increases.
When all the wavelengths of the visible spectrum are combined, they produce white light. This additive mixture of different colors creates the perception of white in our eyes and brain.
Additive and Subtractive Color Mixing
There are two primary ways that colors can be mixed: additive mixing and subtractive mixing.
Additive mixing involves the combination of different wavelengths of light. When red, green, and blue light are shone together, our eyes perceive this as white light. Computer and TV screens use this method to produce colors. The primary colors in additive mixing are red, green, and blue.
| Additive Color Mixing |
|---|
| Red + Green = Yellow |
| Red + Blue = Magenta |
| Green + Blue = Cyan |
| Red + Green + Blue = White |
Subtractive mixing uses pigments and dyes to absorb some wavelengths and reflect others. The primary colors are cyan, magenta, and yellow. When these colors are mixed together, they produce black by absorbing all wavelengths of light.
| Subtractive Color Mixing |
|---|
| Cyan + Magenta = Blue |
| Cyan + Yellow = Green |
| Magenta + Yellow = Red |
| Cyan + Magenta + Yellow = Black |
In subtractive color mixing, combining all colors produces black, while in additive mixing, combining colors produces white light.
Perception of White Light
Although white light is made up of the entire visible spectrum, our eyes and brain do not detect the individual wavelengths. Instead, we perceive white light as its own unique color.
There are a few theories as to why we see white instead of a range of colors:
– Trichromatic theory: The human eye has three types of cone photoreceptor cells that respond to red, green and blue wavelengths. White activates all three receptor types approximately equally, so the brain perceives it as a separate color.
– Opponent process theory: This proposes color vision processing has two stages. First, the cones respond to red, green and blue. Then these signals are processed by retinal ganglion cells that respond to pairs of colors (red-green, blue-yellow, black-white). The white perception arises from the black-white system.
– Reflectance theory: White arises from reflectance of all visible wavelengths off a surface. The balance of wavelengths gives the perception of white compared to colored surfaces that selectively reflect certain wavelengths.
No matter the exact mechanism, the combination of visible spectrum wavelengths from 400-700 nm allows humans to perceive white as its own distinct color.
White Light Composition
Although white contains the full spectrum of visible light, not all white light sources contain the wavelengths in equal amounts. The composition of wavelengths that make up white light can vary.
Some examples of the spectral composition of white light sources:
| Light Source | Spectral Composition |
|---|---|
| Noon sunlight | Relatively balanced spectrum with a peak in the green-yellow region. |
| Incandescent bulb | Strong emission in the red and infrared with weaker blue emission. |
| LED | Blue or UV chip with yellow phosphor coating to create a white appearance. |
| Fluorescent | Spectrum depends on the phosphors used but often peaks in green, red and blue. |
Even though the distributions vary, our eyes and brain sum these spectral inputs to create the perception of white if the right balance is achieved.
White Light in Nature
White light occurs naturally from many sources in the world around us:
– Sunlight: The sun emits light with a full spectrum of visible wavelengths, which appear white to our eyes due to the balance of colors. The sun itself has a surface temperature of 5500°C which produces radiation across the electromagnetic spectrum.
– Lightning: The enormous energy and heat of lightning causes the emission of visible light. The combination of wavelengths again appears white. Interestingly, lightning can also appear blue or purple at the outset due to ionization of air molecules.
– Stars: Extremely hot stars emit radiation across the full visible spectrum which makes them appear white to the human eye. Cooler stars appear more red or orange as the peak of emitted wavelengths shifts down.
– Moonlight: Sunlight is reflected off the moon’s grayish surface back to Earth. The balance of reflected wavelengths gives moonlight a white quality, though slightly muted compared to sunlight.
– Foam & Spray: Bubbles and droplets in ocean surf, waterfalls, and other foamy water reflect a blend of visible wavelengths that make these features appear white.
So in nature, white often arises from thermal radiation, molecular emission, or reflection that contains a fairly balanced share of visible spectrum wavelengths.
White Objects
In addition to white light sources, many objects and materials in our daily life appear white due to how they reflect and scatter light. Some examples include:
– Paper reflects most visible light wavelengths evenly giving it a white appearance. The cellulose fibers scatter light randomly in all directions.
– Snow’s ice crystals have varied shapes and orientations that scatter visible light in all directions, making snow appear white.
– Milk is composed of a suspension of particles and droplets that effectively scatter light without absorbing particular wavelengths.
– Sugar or salt grains look white because the crystalline surfaces reflect and spread out the light.
– White paint contains pigments like titanium dioxide that reflect a broad range of visible wavelengths, obscuring the paint’s base color.
– Clouds form from tiny water droplets or ice crystals suspended in the air. These scatter sunlight equally creating a white appearance.
Whiteness in objects occurs due to diffuse reflection and scattering of light by particles or surface features that don’t preferentially absorb any visible wavelengths. This makes the object look white from all viewing angles.
White Light and Color Perception
White light plays an important role in how we perceive color. As discussed above, white provides maximum reflectance or radiation at all visible wavelengths to the eye. This serves as a reference point for our color vision.
Surfaces or materials that selectively reflect only certain wavelengths are perceived as colored by contrast to a white reference. For example, a leaf appears green because it absorbs red and blue light while reflecting mostly green. Compared to the white light reference, we see just the one color reaching our eyes.
White light also desaturates or dilutes colors through a phenomenon called color constancy. In dim lighting, colors may appear more saturated than they do in bright sunlight. This is because white light reflected off surrounding surfaces mixes with the inbound colored light to our eyes. This has the effect of reducing color intensity or saturation.
So white light provides the benchmark of maximum reflectance that allows our visual system to interpret wavelength-selective absorption or reflection from objects as color. It also interacts with ambient light to moderate color saturation. Without white light as a reference point, our color vision system could not function properly.
Applications of White Light
The unique properties of white light make it useful for many scientific and industrial applications:
– Photography: White light provides even illumination across the visible spectrum allowing accurate reproduction of color. The color temperature of photo lighting must match white balance settings.
– Displays: LCD and OLED screens use RGB pixel elements that combine to produce images in full color under white backlighting. Better white balance improves color accuracy.
– Medical: White LED lighting is used in endoscopes to provide consistent full spectrum illumination of internal tissues for examination and surgery.
– Forensics: Handheld alternate light sources compare a suspect’s fluorescence under UV to white light to identify bodily fluids or forgeries.
– Microscopy: Broad spectrum white light helps ensure suitable contrast, clarity and color rendering of microscopic structures.
– Color matching: Whiteness provides a baseline for color matching and quality control in manufacturing processes like textile dying and automotive painting.
– Agriculture: Full spectrum grow lights with a balanced white output promote healthy growth and development of plants indoors.
From entertainment displays to medical diagnostics to plant growth, white light plays a vital role in color-critical processes and applications across many industries.
Conclusion
White light contains the entire spectrum of visible wavelengths from 400 to 700 nanometers. This combination activates our eye’s cone receptors and color vision pathways in a way that is perceived as white by the visual system. While white light sources have varying spectral composition, the balance of primary colors of red, green and blue is what gives the sensation of whiteness. Whether it originates from sunlight, lightning, or light bulbs, white light allows us to see the world around us in all its colorful splendor. So in a sense, white light is the coming together of all the colors of the rainbow.
