Why can't human eyes detect all wavelengths?

Why can't human eyes detect all wavelengths (of light)? Someone named Fred wrote to ask me this question and I think the answer is of general interest.

The simple answer  is that we are evolved from ancestors who lived in the ocean. Water blocks out all but two small windows of the EM spectrum, and receptors in our eyes are most sensitive to the wavelengths not blocked by water. The physical properties of our eyes also block some wavelengths to protect us from damage.

The structure of the mammalian eye owes itself completely to the task of focusing light onto the retina. This light causes chemical changes in the photosensitive cells of the retina, the products of which trigger nerve impulses which travel to the brain.

In the human eye, light enters the pupil and is focused on the retina by the lens. Light-sensitive nerve cells called rods (for brightness), cones (for color) and non-imaging ipRGC (intrinsincally photosensitive retinal ganglion cells) react to the light. They interact with each other and send messages to the brain. The rods and cones enable vision. The ipRGCs enable entrainment to the earth's 24-hour cycle, resizing of the pupil and acute suppression of the pineal hormone melatonin. - wiki

All organisms are restricted to a small range of the electromagnetic spectrum; this varies from creature to creature, but is mainly between 400 and 700 nm^[20]. This is a rather small section of the electromagnetic spectrum, probably reflecting the submarine evolution of the organ: water blocks out all but two small windows of the EM spectrum, and there has been no evolutionary pressure among land animals to broaden this range.^[21]

The most sensitive pigment, rhodopsin, has a peak response at 500 nm.^[22] Small changes to the genes coding for this protein can tweak the peak response by a few nm;^[2] pigments in the lens can also "filter" incoming light, changing the peak response.^[2]

Most organisms with color vision are able to detect ultraviolet light. This high energy light can be damaging to receptor cells. With a few exceptions (snakes, placental mammals), most organisms avoid these effects by having absorbent oil droplets around their cone cells. The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light — this precludes the possibility of any UV light being detected, as it does not even reach the retina.^[22]:309

Prolonged hours of surfing on the Internet is a major concern that can affect the eyes significantly. White backgrounds on computer screens with a viewing distance of less than 14 inches is known to increase strain, mental fatigue and temporary di-chromatic visions in a normal healthy human being. Trying to opt for black or any non-white backgrounds can help in reducing eye strain infront of PCs. While Black will also eat up the evergy consumption of your computer screen, Green is recommended as the best option to use as backgrounds in webpages. ( color code # 2F6533 is the right median with soothing wavelength of 540nm.) - wiki

Let's get back to why we can only see some wavelengths. The retina has rod cells and cone cells. The rods are for seeing very dim objects and the cones allow us to see colors.

Cones are less sensitive to light than the rod cells in the retina (which support vision at low light levels), but allow the perception of color. They are also able to perceive finer detail and more rapid changes in images, because their response times to stimuli are faster than those of rods.^[2] Because humans usually have three kinds of cones, with different photopsins, which have different response curves, and thus respond to variation in color in different ways, they have trichromatic vision. Being color blind can change this, and there have been reports of people with four or more types of cones, giving them tetrachromatic vision.

image: Normalized responsivity spectra of human cone cells, S, M, and L types

Humans normally have three kinds of cones. The first responds most to light of long wavelengths, peaking in the yellow region; this type is designated L for long. The second type responds most to light of medium-wavelength, peaking at green, and is abbreviated M for medium. The third type responds most to short-wavelength light, of a violet color, and is designated S for short. The three types have peak wavelengths near 564–580 nm, 534–545 nm, and 420–440 nm, respectively.^[3][4] The difference in the signals received from the three cone types allows the brain to perceive all possible colors, through the opponent process of color vision.

The color yellow, for example, is perceived when the L cones are stimulated slightly more than the M cones, and the color red is perceived when the L cones are stimulated significantly more than the M cones. Similarly, blue and violet hues are perceived when the S receptor is stimulated more than the other two.

The S cones are most sensitive to light at wavelengths around 420 nm. However, the lens and cornea of the human eye are increasingly absorbative to smaller wavelengths, and this sets the lower wavelength limit of human-visible light to approximately 380 nm, which is therefore called 'ultraviolet' light. People with aphakia, a condition where the eye lacks a lens, sometimes report the ability to see into the ultraviolet range.^[5] At moderate to bright light levels where the cones functions, the eye is more sensitive to yellowish-green light than other colors because this stimulates the two most common of the three kinds of cones almost equally. At lower light levels, where only the rod cells function, the sensitivity is greatest at a blueish-green wavelength.

If you want to get a little deeper, you might wonder how light entering the eye is turned into signals which are sent to our brains. HowStuffWorks has a nice write up:

When light enters the eye, it comes in contact with the photosensitive chemical rhodopsin (also called visual purple). Rhodopsin is a mixture of a protein called scotopsin and 11-cis-retinal -- the latter is derived from vitamin A (which is why a lack of vitamin A causes vision problems ... [When severe vitamin A deficiency is present, then night blindness occurs. ] ). Rhodopsin decomposes when it is exposed to light because light causes a physical change in the 11-cis-retinal portion of the rhodopsin, changing it to all-trans retinal. This first reaction takes only a few trillionths of a second. The 11-cis-retinal is an angulated molecule, while all-trans retinal is a straight molecule. This makes the chemical unstable. Rhodopsin breaks down into several intermediate compounds, but eventually (in less than a second) forms metarhodopsin II (activated rhodopsin). This chemical causes electrical impulses that are transmitted to the brain and interpreted as light.  ...

Activated rhodopsin causes electrical impulses in the following way:

1. The cell membrane (outer layer) of a rod cell has an electric charge. When light activates rhodopsin, it causes a reduction in cyclic GMP, which causes this electric charge to increase. This produces an electric current along the cell. When more light is detected, more rhodopsin is activated and more electric current is produced.


2. This electric impulse eventually reaches a ganglion cell, and then the optic nerve.


3. The nerves reach the optic chasm, where the nerve fibers from the inside half of each retina cross to the other side of the brain, but the nerve fibers from the outside half of the retina stay on the same side of the brain.


4. These fibers eventually reach the back of the brain (occipital lobe). This is where vision is interpreted and is called the primary visual cortex. Some of the visual fibers go to other parts of the brain to help to control eye movements, response of the pupils and iris, and behavior.

Color Vision

The color-responsive chemicals in the cones are called cone pigments and are very similar to the chemicals in the rods.... There are three kinds of color-sensitive pigments:

• Red-sensitive pigment


• Green-sensitive pigment


• Blue-sensitive pigment

Each cone cell has one of these pigments so that it is sensitive to that color. The human eye can sense almost any gradation of color when red, green and blue are mixed.

What happens when a color pigment in a cone in your retina absorbs a photon of light with a particular wavelength? How does this cause a reaction which sends a signal, exactly?

Light is a form of electromagnetic radiation.

"In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy, for example, to heat."

When light is absorbed by molecules, the electronics in those molecules make a quantum leap to a higher energy state. This temporary state change of the electron(s) makes the changed molecules attract differently the other molecules nearby. This causes a chain reaction which, in the cones of your eye, leads to signals being sent to your brain.

If you are still curious, you could dig deeper into how, at the molecular level, photons with certain wavelengths are absorbed by our different color pigments.