Ultraviolet light, commonly referred to as UV light, occupies wavelengths on the electromagnetic spectrum that are shorter than visible light but longer than X-rays. While UV light is invisible to the human eye, it can sometimes appear to have a violet or purple glow. This has led to the common perception that UV light is purple. However, the actual color of UV light depends on several factors.
The Visible Spectrum
To understand if UV light is always purple, we first need to understand visible light. Visible light that humans can see makes up only a small portion of the full electromagnetic spectrum. The visible spectrum ranges from violet light with wavelengths around 380-450 nm to red light with wavelengths around 620-750 nm. Other colors like blue, green, yellow, and orange fall between these ranges.
Ultraviolet light sits next to the visible violet end of the spectrum. It is commonly divided into three regions:
- UVA: 315-400 nm
- UVB: 280-315 nm
- UVC: 100-280 nm
So while parts of the UVA spectrum come close to the visible violet wavelengths, most UV light actually has a shorter wavelength than anything humans can see.
Why UV Light Sometimes Looks Purple
Even though UV wavelengths are outside the visible spectrum, they can sometimes appear to glow purple or violet when observed. There are a couple reasons why this occurs:
- Some UV lamps or LEDs emit a small amount of visible light in addition to the UV. This visible component can look purple.
- Some materials will fluoresce or phosphoresce when exposed to UV, emitting visible violet or purple light.
- The human eye does not have a sharp cutoff in sensitivity at 380 nm. We still perceive some wavelengths between 380-400 nm as a deep violet color.
So while pure UV light itself is invisible, the glow we sometimes think of as “ultraviolet” is actually visible violet light produced as a side effect of the UV emission. This leads to the association of UV with the color purple.
Measuring UV Light
To truly determine the color of UV light, we need instruments that can measure wavelengths beyond what our eyes can see. Spectrometers are tools that can measure the intensity of light across a wide range of wavelengths.
For example, here are spectral plots of common light sources and how they compare to the visible spectrum:
| Light Source | Visible Spectrum | UV Spectrum |
|---|---|---|
| Sunlight |  |
 |
| Fluorescent Light |  |
 |
| LED Flashlight |  |
 |
These plots show that while the visible spectrum may look white or purple-tinted, the UV spectrum is colorless. The intensity at various UV wavelengths can be precisely measured.
True Color of UV Light
So what color is ultraviolet light really? The answer is that UV wavelengths are outside the range of normal human vision. Pure UV light has no inherent color at all.
The purple glow we sometimes associate with UV is caused by fluorescence or phosphorescence of materials in the visible spectrum. It is not the actual color of UV photons themselves. A spectrometer reveals that UV spans a wide range of wavelengths and intensities, rather than a single color.
However, because this purple glow is the only way our eyes can perceive UV radiation, it is reasonable to describe UV light as purple for general purposes. But technically, UV has no color, and many UV sources emit no visible light at all. The vibrant purple glow is not inherent to ultraviolet wavelengths themselves.
Uses for Ultraviolet Light
Even though it is invisible, ultraviolet light has many useful applications that rely on its unique properties:
- Disinfecting surfaces and water using germicidal UV lamps
- Detecting fluorescent dyes and minerals with blacklights
- Curing UV-sensitive adhesives, paints, and varnishes
- Analyzing minerals and detecting forgeries under UV illumination
- Attracting insects to bug zappers
- Creating vitamin D in skin via exposure to UVB in sunlight
The ability of UV light to damage DNA can be harnessed for sterilization, but also poses risks like sunburn, skin cancer, and eye damage. Protective measures are needed when working with UV sources.
Conclusion
While ultraviolet light appears purple when observed through fluorescence, its actual wavelengths have no inherent color. The glowing purple is caused by UV photons exciting materials to emit visible violet light. Measuring UV with spectrometers reveals a broad, colorless spectrum of wavelengths shorter than visible light. So technically, UV light itself is not purple – the purple glow requires additional materials to produce it.