Visible light is the portion of the electromagnetic spectrum that is visible to the human eye. The frequency of visible light ranges from about 4.3 x 10^14 Hz to 7.5 x 10^14 Hz, corresponding to wavelengths of 700 nm (red) to 400 nm (violet). However, the limits of human vision do not align perfectly with these boundaries. This article will explore the lowest frequency (longest wavelength) light that is visible to humans.
The Electromagnetic Spectrum
The electromagnetic spectrum encompasses all frequencies of electromagnetic radiation. The spectrum can be divided into different regions based on the frequency and wavelength, including (from high to low frequency):
- Gamma rays
- X-rays
- Ultraviolet
- Visible light
- Infrared
- Microwaves
- Radio waves
The visible light portion falls between ultraviolet and infrared light. It corresponds to wavelengths from approximately 400-700 nanometers. The frequency ranges from about 4.3 x 10^14 Hz to 7.5 x 10^14 Hz. Visible light can be further divided into spectral colors:
- Violet: 380-450 nm
- Blue: 450-495 nm
- Green: 495-570 nm
- Yellow: 570-590 nm
- Orange: 590-620 nm
- Red: 620-750 nm
The ranges for visible light are commonly cited as 400-700 nm or 430-750 nm. However, in reality, the limits of human vision do not correspond exactly to these numbers.
Limits of Human Vision
The theoretical limits of the visible spectrum do not perfectly match our visual perception. This is because the light-sensitive cells in our eyes, known as cone cells, do not respond equally to all wavelengths. There are three types of cone cells – short, medium and long wavelength cones. Their peak sensitivities correspond roughly to blue, green and red light.
Beyond about 700 nm, our vision drops off quite quickly. This is because the long wavelength “red” cones lose sensitivity in the far red and infrared. However, we retain some visual response below 400 nm into the ultraviolet. The short wavelength “blue” cones have sensitivity down to about 350-370 nm.
Therefore, the functional limit of human vision is not a hard cutoff at 400 or 700 nm. A more accurate range would be about 350-750 nm, extending slightly beyond the commonly cited visible light boundaries.
Low Frequency Cutoff
What determines the lowest frequency or longest wavelength of light visible to humans? As mentioned, our long wavelength “red” cone cells lose sensitivity rather quickly above 700 nm. However, some studies have shown that we retain a small degree of visual response beyond 700 nm.
For example, one study showed that people are able to detect wavelengths up to about 740-780 nm when the light intensity is high. Though the response is weak, this indicates our eyes do have some sensitivity in the far red part of the spectrum.
When observing dim light under dark-adapted conditions, such as under a starry sky, the sensitivity appears to extend farther. Wavelengths up to about 820-850 nm may be weakly visible under these conditions. Essentially, the lower limit of human vision depends on the intensity of the light.
Based on available evidence, conservative estimates put the functional low frequency cutoff for human vision around 700-740 nm. However, under certain conditions, wavelengths up to 850 nm may potentially be spotted. This infrared light is literally on the edge of our visual perception.
Extending Vision Beyond Normal Limits
While practical human vision may cut off between 700-850 nm, there are ways to extend your perception beyond the normal limits. These include:
- Afterimages – Staring briefly at an intense red light can produce an afterimage allowing you to see infrared light.
- Phosphenes – Applying mechanical, electrical or magnetic stimulation to the eye can generate extra-spectral phosphenes, flashes of light at the edges of our vision.
- Dietary manipulation – Consuming certain carotenoid and vitamin A rich supplements may enhance night vision and edge of spectrum perception.
However, these effects are typically weak and temporary. The fundamental limits of our visual perception are constrained by the sensitivities of our photoreceptor cells. Engineering permanent infrared night vision would require augmenting the human eye with additional receptor types.
Cutoffs in Other Species
While humans normally cannot see infrared, some other species can. For example:
- Snakes – Pythons and pit vipers have receptors sensitive to far infrared radiation to detect prey.
- Insects – Many insects including bees and ants can see light in the near UV and IR wavelengths invisible to humans.
- Shrimp – Mantis shrimp have 12 photoreceptor types spanning an enormous range from deep UV to far red.
Having photoreceptors specialized for different portions of the electromagnetic spectrum allows these species to sense their environments in ways humans cannot. Every species is limited by its own specific range of vision.
Practical Limits for Human Vision
Based on available evidence, the practical low frequency cutoff for human vision is around 700 nm. This corresponds to a frequency of about 4.3 x 10^14 Hz. We have very little visual response below this level, though occasional detection of wavelengths up to 850 nm may be possible in some cases.
For comparison, the theoretical visible light boundary is commonly defined as starting around 400-430 nm. This corresponds to frequencies of around 7.5-7.9 x 10^14 Hz on the high end of the visible spectrum. In practice, we can see wavelengths down to about 350 nm in this range under ideal conditions.
So while visible light is commonly defined from 400 to 700 nm, the functional boundaries of human vision are 350 to 700/850 nm, with very little response outside this window. Going beyond our biological constraints would require augmenting our eyes with enhanced photoreceptors.
Summary
- Visible light is usually defined as 400-700 nm, but human vision does not follow this exactly.
- Our visual response drops off quickly above 700 nm, but some sensitivity may persist up to 850 nm.
- Practical low end cutoff is around 700 nm (4.3 x 10^14 Hz) , though occasional 850 nm detection is possible.
- We retain UV response down to about 350 nm at the high frequency end.
- True visible cutoff likely 350-750 nm, depending on conditions.
- Other species see more of the spectrum – UV, infrared and beyond.
In summary, the lowest frequency or longest wavelength of light visible to normal human vision is around 700 nm, though perception may occasionally persist up to 850 nm in some circumstances. This infrared edge lies just beyond our nominal biological red limit, but still falls far short of what other species can detect.
Conclusion
The lowest frequency electromagnetic radiation visible to the human eye corresponds to a wavelength of approximately 700 nanometers (in the red range). This equates to a frequency of around 4.3 x 10^14 Hz. While the commonly cited lower boundary of visible light is 400 nm (7.5 x 10^14 Hz), human visual response falls off quickly past 700 nm. We retain some sensitivity up to about 850 nm under certain conditions, but lack sufficient photoreceptor response below this infrared threshold. Other species adapted to sensing specific portions of the EM spectrum can see far beyond the human visual cutoff. But for our eyes, the low frequency limit of perception hovers around 4.3 x 10^14 Hz, right at the edge between visible red and invisible infrared.
