What is the visible light spectrum and can the human eye see it?
The visible light spectrum is a portion of the electromagnetic spectrum picked up by the human eye.
This spectrum ranges between 380 and 740 nanometres by wavelength, and between 430 and 770 Terahertz in frequency.
The visible light spectrum makes up 100% of all light humans are able to observe.
How is the visible light spectrum detected?
The visible light spectrum is detected is via a triplicate of cone cells and rod cells, as well as intrinsically photosensitive retinal ganglion cells.
These three types of cells are classified as photoreceptor cells due to the presence of proteins which absorb photons of specific wavelengths.
This triggers a change in the cells’ overall electrical charge, otherwise known as membrane potential.
The change in potential is then carried to the brain for processing into an image.
What are rod cells?
While highly sensitive to photons, rod cells do not differentiate greatly between wavelengths of light and colour.
They are primarily used as a night-vision aid and of course, as the name would imply, they are shaped like rods.
What are cone cells?
Cone cells transmit a different membrane potential dependent on the wavelength of the photon it has absorbed.
They function optimally in high-intensity light, such as during the day.
Cone cells are less sensitive to photons than rod cells, and this is expressed through the pupil dilating during low-light situations to allow additional photons into the cornea for detection.
These cells are very tiny and densely packed into the eye to maximise cells over a relatively small area, such as the human eye.
What are intrinsically photosensitive retinal ganglion cells?
Intrinsically photosensitive retinal ganglion cells are a type of cell found deeper in eye tissues than the cones and rods, which are placed on the inner surface.
While these cells can detect light from the visible light spectrum, the processing of information is not based upon image formation like the rod and cone cells.
Instead, they are more towards aiding and abetting the flow into human's circadian rhythm (the part of your brain called the hypothalamus).
These cells contribute to the photon-based release of melatonin, a hormone which aids sleeping, as well as sending information to the brain which controls our circadian rhythm.
While this range of electromagnetic radiation is what we humans can see, it is actually unique for all organisms with eyes.
Bees, for example, see the world in UV-light, which is outside of the spectrum available for human eyes to detect.
These differences are mainly due to the evolutionary requirements of a particular species.