Human vision is limited to a very small portion of the electromagnetic spectrum. The visible spectrum, as perceived by the human eye, corresponds to wavelengths from approximately 380 to 740 nanometers. This equates to a frequency range of 405–790 terahertz. However, the full electromagnetic spectrum extends from below the frequencies and wavelengths of radio waves to above the frequencies and wavelengths of gamma rays, spanning more than 25 orders of magnitude in frequency and wavelength. There are many forms of electromagnetic radiation that are imperceptible to humans due to the limitations of our visual system, but are detectable with specialized instruments. Here we explore some of the main types of light that lie outside the human visible range.
Infrared Light
Infrared radiation lies between the red end of the visible spectrum and terahertz radiation. It has longer wavelengths and lower frequencies than visible light. While humans cannot see infrared waves, we can detect them as heat. The infrared spectrum is often divided into three regions:
Near-Infrared
The near-infrared region is the closest to visible light and extends from about 750 nm to 1.4 μm. Near-infrared can be detected by digital cameras and is used in fiber optic telecommunication. Though invisible to humans, many birds, insects, and snakes can see near-infrared.
Mid-Infrared
The mid-infrared region covers wavelengths from 1.4 μm to 3 μm. These waves are used in thermal imaging because mid-infrared waves are emitted by objects at room temperature. Night vision goggles detect this infrared radiation to create images.
Far-Infrared
Far-infrared lies between about 3 μm and 1 mm. far-infrared waves are felt as heat and are used in thermal imaging to detect heat loss in buildings and to monitor human and animal activity. Far-infrared heating systems make use of these waves’ ability to be absorbed by human skin.
So while humans cannot directly see any infrared light, we can detect its presence through its heating effect. Many modern technologies rely on infrared radiation for thermal imaging, night vision, and telecommunications.
Ultraviolet Light
Ultraviolet (UV) light has shorter wavelengths and higher frequencies than violet, the highest frequency visible to humans. Though UV waves are damaging to human eyes and skin at high intensities, some UV is important for health, enabling the production of vitamin D, supporting bone health, and killing germs. There are three types of UV radiation:
UVA
UVA has the longest wavelengths and lowest frequency of UV light. These 315-400 nm waves can penetrate deep into the skin and are associated with skin aging and wrinkling from sun exposure. They can also cause DNA damage that results in skin cancer.
UVB
UVB waves range from 280-315 nm. They are more energetic than UVA so can penetrate less deeply into skin, but are directly absorbed by DNA molecules in skin cells. UVB exposure causes sunburns and is the main cause of skin cancer. It also enables the production of vitamin D.
UVC
UVC has the shortest wavelengths from 100-280 nm and is extremely dangerous, but doesn’t reach the Earth’s surface as it is absorbed by the ozone layer. It is used in germicidal lamps to disinfect surfaces and kill bacteria.
So while humans cannot see UV light directly, overexposure from the sun can damage skin and eyes. However, some UV exposure is necessary for vitamin D and killing harmful microbes.
X-Rays and Gamma Rays
X-rays have wavelengths from about 0.01 nm to 10 nm, corresponding to frequencies of 3×1016 Hz to 3×1019 Hz. They have much higher energy than UV and visible light. Due to this energy level, X-rays can penetrate through soft tissues and are used to create images of dense structures like bones.
Gamma rays have the shortest wavelengths and highest frequencies on the electromagnetic spectrum. Natural gamma rays are emitted from radioactive decay processes, while artificial gamma rays are produced from nuclear reactions or the decay of accelerated atomic particles.
The extreme energy of X-rays and gamma rays make them hazardous. Exposure can damage and kill cells. However, their ability to pass through matter is invaluable for medical imaging and treatments, such as sterilizing medical equipment.
So although human eyes cannot detect them, x-rays and gamma rays are useful for medical applications when properly controlled. Their ability to harm cells also enables their use in radiation therapy for cancer treatment.
Detection Methods
| Radiation Type | Detection Methods |
|---|---|
| Infrared | Thermal cameras, infrared detectors, night vision devices |
| Ultraviolet | UV meters, fluorescent dyes |
| X-rays | Photographic films, ionization detectors, scintillators |
| Gamma rays | Geiger counters, scintillation counters |
Natural Sources
In addition to artificial sources, many invisible forms of light are generated naturally:
– Infrared radiation is emitted by all objects above absolute zero through blackbody radiation
– UV is emitted from the Sun with intensity and wavelengths dependent on factors like sunspot cycles
– Some compounds fluoresce UV light when absorbing visible light
– X-rays and gamma rays are emitted from radioactive decay, cosmic radiation, stars, and some celestial events like neutron star collisions
So while humans cannot sense these natural processes, we can detect the invisible light using technology.
Conclusion
The human eye can only see a tiny window into the full electromagnetic spectrum. Many other forms of light and radiation exist below the visible frequencies of infrared and above the visible frequencies of ultraviolet. High energy x-rays and gamma rays also have smaller wavelengths than the human visual range. So there are many types of light that humans simply cannot see directly. However, through clever use of technology we can visualize and take advantage of these unseen rays. Infrared cameras, ultraviolet meters, and x-ray imaging all allow us to detect and utilize radiation that is imperceptible to our eyes. So although we cannot see all forms of light, we can still find ways to observe and harness them for our benefit.