Infrared light is a type of electromagnetic radiation that is invisible to the human eye. It has longer wavelengths than visible light and sits just outside the red end of the visible light spectrum. Because we can’t see infrared light directly, it doesn’t have an associated color that our eyes perceive. However, we can determine what visible color is closest to infrared in terms of wavelength.
The Electromagnetic Spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. It includes radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays. Visible light makes up only a small portion of the full spectrum, from approximately 380 nanometers to 740 nanometers in wavelength. Infrared light sits just next to the red end of the visible spectrum, starting around 700 nanometers and extending to 1 millimeter in wavelength.
The visible colors we see result from different wavelengths of light interacting with our eyes. Red light has the longest visible wavelengths, while violet and blue light have the shortest. Infrared wavelengths are longer than red, so the closest visible color to infrared in terms of wavelength is red.
Perceived Color from Infrared Radiation
Although infrared is invisible to our eyes, we can sometimes perceive a color associated with intense infrared radiation. For example, when looking at extremely hot objects such as red-hot metal or the heating elements in a toaster, we may see a faint red glow. This is caused by the object emitting infrared radiation strongly peaked in the near-infrared, which stimulates our red color receptors slightly.
So while infrared itself has no distinct color, the color our eyes register from strong infrared emission is a deep, dark red. Essentially, the hottest visible red shade is the closest our eyes can perceive to the actual color of infrared light.
Why Infrared is Invisible
Infrared radiation is invisible for a couple key reasons:
- Infrared wavelengths are too long for our eyes to detect – our visible color receptors (cones) are sensitive only to visible light waves between 380-740nm.
- Infrared does not contain enough energy at each photon to stimulate a response in our eye. Visible color is determined both by wavelength and energy per photon.
Our eyes can register faint responses to intense near-infrared sources, allowing us to perceive a deep red color. But the vast majority of infrared radiation, with wavelengths from 700nm up to 1mm, remains completely invisible to human sight.
Properties of Infrared Light
Infrared light shares many properties with visible light, just with longer wavelengths. A few key properties include:
- Infrared travels at the speed of light – 299,792 km/s.
- Infrared exhibits wave-particle duality – it acts as both a particle (photon) and a wave.
- Infrared can be reflected, refracted, and absorbed like visible light.
- Infrared can transfer heat and energy when absorbed by objects.
However, infrared does have some unique properties compared to visible light. These include:
- Infrared is not detected by the human eye.
- Infrared has longer wavelengths than any visible color (380-700nm).
- Infrared is strongly emitted by warm and hot objects through thermal radiation.
- Infrared can pass through some materials that visible light cannot.
Infrared Detection and Use
Although we cannot see it, infrared radiation is very useful and used in many applications:
- Thermal imaging – Sensors can detect infrared radiation to see variations in heat and temperature.
- Night vision – Near-infrared amplifies visible light to see in very low light conditions.
- Tracking – IR lasers and sensors are used for motion tracking and object detection.
- Communications – IR can be used for short-range data transmission between devices.
- Spectroscopy – Chemical composition can be determined by analyzing infrared absorption signatures.
Detection methods for infrared utilize advanced cameras, sensors, and optical materials that can register the heat and light outside our visual range.
Closest Visible Colors to Key Infrared Wavelengths
Different infrared wavelengths crossover into the visual spectrum at different visible colors:
| Infrared Wavelength | Closest Visible Color |
|---|---|
| 700-890nm | Red |
| 890-1,200nm | Dark red verging on invisible |
| 1,200-1,400nm | Invisible |
So in summary, the visible color we perceive as closest to infrared is a deep, dark red around 700nm. As the wavelength increases further from 700nm, infrared progressively becomes completely invisible to our eyes.
Conclusion
Although infrared light exists outside the visible spectrum, the closest visible color to it in terms of wavelength is red. Specifically, a deep dark red around 700nm is the last shade our eyes can still perceive before infrared transitions to being completely invisible. This is because longer infrared wavelengths do not contain enough energy to stimulate our visual color receptors. So while infrared itself has no true color, red is the closest approximation our eyes can see of this band of invisible light.
