White light is made up of all the colors of the visible light spectrum. When all the wavelengths of visible light are present, our eyes see this combination of light as white. Two common ways to produce white light are by combining blue light and yellow light, or by combining red, green, and blue light. In this article, we’ll focus on how combining blue and yellow light creates white light.
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 violet light with a wavelength of about 380 nanometers to red light with a wavelength of about 740 nanometers. The colors we see result from different wavelengths of light. Here is an overview of the visible spectrum:
| Color | Wavelength Range |
|---|---|
| Violet | 380-450 nm |
| Blue | 450-495 nm |
| Green | 495-570 nm |
| Yellow | 570-590 nm |
| Orange | 590-620 nm |
| Red | 620-750 nm |
Additive Color Mixing
When it comes to light, the primary colors are red, green, and blue. This is known as an additive color model. With additive color mixing, combining primary colors of light results in secondary colors.
The primary additive colors:
– Red light
– Green light
– Blue light
The secondary additive colors produced by mixing pairs of primaries:
– Red + Green = Yellow
– Red + Blue = Magenta
– Green + Blue = Cyan
All three primary colors together produce white light. This is additive color mixing – combining light creates more colors. Computer and TV screens use the additive RGB color model to produce colors by mixing red, green and blue light.
Why Blue and Yellow Make White
As seen above, when red, green, and blue light are combined, the result is white light. However, white light can also be produced by mixing just two colors – blue light and yellow light.
Blue light has wavelengths around 450-495 nm. Yellow light has wavelengths around 570-590 nm. When these two colors are mixed, the resulting light stimulates both the blue and yellow color receptors in our eyes.
Our eyes have three types of color receptors called cone cells:
– Red cones that detect long wavelengths (620-750 nm)
– Green cones that detect medium wavelengths (495-570 nm)
– Blue cones that detect short wavelengths (450-495 nm)
Since blue light activates the blue cones and yellow light activates both the green and red cones, combining these two colors produces a response from all three cone cell types. This full stimulation of all the eye’s cone cells is perceived as white light by the brain.
The Science Behind Blue and Yellow Light Mixing
The reason blue and yellow light combine to make white light can be explained both by the biology of human color vision, and by the physics of light wavelengths.
Human Color Perception
As mentioned, the three types of cone cells in our eyes allow us to detect red, green and blue light. These cone cells contain photopigments that are sensitive to particular wavelengths. When these photopigments absorb light, they trigger neural signals to the brain. The brain interprets the signals from all three cone cell types together as different colors.
When blue light (around 450-495 nm) and yellow light (around 570-590 nm) enter the eye together, they fully stimulate the blue cones, green cones, and red cones. The brain receives a full set of signals from the cone cells, which it perceives as white light. No cone cell type is lacking stimulation, so the light is not interpreted as a partial color.
Additive Light Wavelengths
In terms of the physics of light, blue light wavelengths (around 450-495 nm) and yellow light wavelengths (around 570-590 nm) fall on opposite ends of the visible spectrum. When these wavelengths mix together, they create a relatively uniform band of wavelengths spanning the full visible spectrum.
This complete band of visible wavelengths is additive white light. While red, green, and blue provide a more even distribution of wavelengths, blue and yellow produce a sufficient spread to be perceived as full spectrum white light.
Other Ways to Produce White Light
Combining blue and yellow light is not the only method for generating white light. Here are a few other ways white light can be produced:
– RGB: Mixing red, green and blue light. This is additive color mixing using the primary colors of light. RGB is used in TVs, computer monitors, and other color displays.
– Paint Pigments: Mixing paint pigments in equal proportions, such as in RGB paint mixing, produces shades of gray or white. This subtractive color mixing uses reflected light.
– LASERs: Some white-light LASERs combine a blue LASER diode with yellow phosphor coating to emit white light. The blue is absorbed and re-emitted as yellow.
– Incandescent Bulbs: Heating a tungsten filament produces a broad continuum of wavelengths that appear white. No phosphors or gasses are needed to produce other colors.
– Fluorescent Bulbs: UV light excites a phosphor coating to emit multiple wavelengths, blending to white. Different phosphors emit different colors across the spectrum.
– LED: White LEDs combine a blue LED with yellow phosphor, similar to white LASERs. The blue light stimulates the phosphor to emit yellow light.
So in summary, the two main ways to make white light are additive color mixing of primary colors (RGB), or combining a shorter blue wavelength with a longer yellow/phosphor emission.
Everyday Examples of Blue and Yellow Light Mixing
We can observe the mixing of blue and yellow light to produce white light in many everyday settings:
– Early Morning/Late Afternoon Sunlight: During these times, sunlight passing through the atmosphere scatters blue light. This mixes with the yellowish sunlight to create white light.
– White LED Bulbs: As mentioned above, most white LED bulbs contain a blue LED combined with yellow phosphor. The blue LED pumps out blue light, while the phosphor re-emits yellow light. Together they appear white.
– Computer/Phone Screens: To produce white pixels on an LCD screen, the blue and yellow sub-pixels are turned on together with the red sub-pixel to create full spectrum white light.
– Magnesium Flames: Burning magnesium produces bright white flames. This is due to blue and yellow spectral emission lines from magnesium atoms. Blue and yellow wavelengths are emitted which mix to white light.
– Printing: Traditional CMYK printing uses cyan (blue), magenta (pink), and yellow ink to produce white paper as a secondary color. The combined reflected colors appear white.
So in many natural and artificial light sources, a combination of blue and yellow light contributes to the perception of white light.
Conclusion
In summary, blue light around 450-495 nm and yellow light around 570-590 nm can combine to produce white light for the human eye due to the biology of our color vision and the physics of additive light mixing. Blue stimulates the short wavelength cones, yellow stimulates the medium and long wavelength cones, and together this full cone cell stimulation is perceived as white light. The wavelengths from blue and yellow span the visible spectrum sufficiently to blend as full spectrum white light. This explains why combining blue and yellow light creates the perception of white, both in manufactured devices like screens and bulbs, as well as in nature during sunrise and sunset.
