At first glance, the question “Can you make black and white light?” may seem nonsensical. After all, light is typically associated with brightness, illumination, and color. However, there are some interesting insights to be gained by exploring this topic further.
To begin, it is important to understand what exactly light is. Visible light is a form of electromagnetic radiation that can be perceived by the human eye. It has wavelengths ranging from about 380 to 740 nanometers. When all wavelengths of visible light are combined together, white light is produced. But light can also be broken down into its composite colors using prisms, filters, or other methods.
The two extremes of the visible light spectrum are red light with long wavelengths (~700 nm) and violet/blue light with short wavelengths (~400 nm). In between these extremes are all the colors of the rainbow – orange, yellow, green, blue, indigo. When light only contains wavelengths from one region of the spectrum, it appears as that particular color.
So where do black and white come into play? Black is technically the absence of visible light. An object appears black when it absorbs all wavelengths of visible light and does not reflect any back to our eyes. White, on the other hand, occurs when an object reflects back all wavelengths of visible light equally. This complete, uniform reflection is interpreted by our eyes as the color white.
Creating Black Light
While true black light sounds like an impossibility, there are a few ways to produce something that approximates black light:
Ultraviolet (UV) light has shorter wavelengths than visible violet light. It sits just outside the visible spectrum, meaning our eyes cannot detect it. However, we can experience UV light indirectly. Many objects will fluoresce or glow when illuminated by UV light.
Black light bulbs rely on this principle. They filter out most visible wavelengths, transmitting primarily long ultraviolet waves of around 365 nm. To our eyes, it simply looks like dim purple light. But fluorescent or phosphorescent material glows brightly under these black light conditions. This effect is used in psychedelic poster art, forensic detection, and finding certain mineral deposits.
In a dark enough environment, a faint red light source can appear black. For example, photographic darkrooms use red safety lights so workers can see but the sensitive photo paper remains unexposed. The low-intensity red light looks black, but enables vision once the eyes adjust.
Similarly, using a heavily filtered flashlight or laser pointer that only emits deep red wavelengths can produce a dim beam that subjectively seems black in a very low ambient light setting. This is not true black light, but it can subjectively appear black under the right conditions.
Blackbody radiation refers to electromagnetic radiation that is emitted by a heated object. The wavelength distribution and intensity of this emitted light depends solely on the temperature of the object.
As a blackbody is heated to progressively higher temperatures, it first glows dull red, then orange, yellow, white, and finally an intense blue. An object at room temperature emits almost no visible light, appearing black. But at higher temperatures it can subjectively produce a black-body glow in the red/orange region of the spectrum.
Creating White Light
Unlike black light, it is quite straightforward to produce light that appears subjectively white to human eyes:
Natural sunlight contains all wavelengths of visible light. This combination of colors appears white to our eyes. Direct unfiltered sunlight provides the original benchmark for white light against which other light sources are compared.
Noon daylight has a color temperature around 5500K. As the sun rises and sets, the light takes on warmer orange and red hues. But at high noon, the solar spectrum provides nearly perfect white light ideal for color rendition.
Incandescent and halogen lights produce thermal radiation by heating a tungsten filament. The hotter the filament, the whiter the light produced. These bulbs emit a continuous spectrum of visible wavelengths that subjectively appears white, although it may be slightly warmer or cooler than daylight.
Incandescent lights were long the most common artificial white light source. But they are inefficient, wasting energy as heat. This led to the phase-out of traditional incandescent bulbs in many countries.
Fluorescent tube lights pass electricity through mercury vapor, which emits ultraviolet light. The UV causes a phosphor coating inside the tube to fluoresce, producing visible light.
Different phosphor compositions can tune the color of the light. Fluorescent tubes with a blend of phosphors can produce a broad emission spectrum that appears subjectively white to our eyes. Cool white fluorescents emit bluer light, while warm white variants have more yellow/orange hues.
LED lights use a semiconductor chip to directly emit visible light. LEDs tailored to emit different wavelengths can be combined to produce composite white light.
Some white LEDs combine a blue LED chip with a yellow phosphor coating, balancing the two colors to achieve white light. More advanced LEDs mix red, green, and blue chips together and tune the intensity of each color channel to optimize the white light output.
Black and White Light in Summary
While pure black light is not possible, several techniques can produce low-intensity red/UV emission that appears black in a dark environment. And a variety of light sources from the sun to LEDs can stimulate our eyes with a balance of visible wavelengths that are perceived as white.
So in summary:
|Black Light Approximations
|True White Light Sources
|Long-wave “blacklight” UV lamps
|Extremely dim red lighting
|Blackbody radiation at low temperatures
While black light itself remains elusive, Subjectively black light effects can be produced using strongly filtered sources in a suitable context. And many natural and artificial light sources are capable of stimulating the full visible spectrum interpreted as white by our eyes. So with the right equipment and settings, forms of black and white light can be generated.
Frequently Asked Questions
Is black light really a thing?
True black light is not possible since light by definition is electromagnetic radiation visible to the eye. However, ultraviolet lamps filtered to emit primarily long UV wavelengths can appear dark purple and subjectively seem black in a darkened environment. Their ability to make fluorescent materials glow demonstrates they are emitting electromagnetic radiation, even if our eyes don’t perceive the UV light directly.
Why does a prism split white light into a rainbow?
A prism refracts different wavelengths of light by different amounts based on their frequency. This dispersion separates the component colors of white light. Longer red wavelengths bend the least while shorter violet/blue wavelengths bend the most. In between, orange, yellow, green and other colors are split out based on their wavelength. This rainbow effect demonstrates that white light is actually a uniform mixture of the visible spectrum.
How are black and white defined in terms of light?
White light occurs when a light source emits a balanced combination of all visible wavelengths that appears uniformly white to our eyes. Black is defined as the complete absence of visible light emission or reflection. A black object absorbs all visible wavelengths, reflecting none back to our eyes. So white light contains all colors equally, while black indicates a total lack of visible light.
Why do some street lights use sodium vapor bulbs?
Sodium vapor lamps emit light concentrated at specific yellow wavelengths. This monochromatic yellow light provides decent visibility for human vision and street lighting applications. But its limited color spectrum provides poor color rendition. Sodium vapor lights are energy efficient and long-lasting, making them a cost-effective (if unappealing) outdoor lighting option. More advanced LED street lights can achieve superior efficiency and color quality.
Can you make white light by combining paint pigments?
While combining all paint pigments together makes black, mixing light colors is additive. Combining pure red, green and blue light makes white light, while mixing all paint pigments makes black. This is because paint pigments absorb certain colors, while light emission combines different colors. So additive light mixing follows different principles compared to subtractive pigment mixing.
In conclusion, while true black light is likely impossible, clever techniques like filtering deep red or long-wave ultraviolet emission can create the illusion of black light. And white light can be produced through thermal radiation, fluorescence, or LEDs that emit a balanced blend of visible wavelengths. So with the right equipment and context, forms of black and white light can be generated, even if pure light versions remain elusive in the strictest sense. The search for black and white light reveals some fascinating properties of light and perception.