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LECTURE 13. LIGHT

Current knowledge is insufficient to determine if anything existed prior to the singularity. Sixteen centuries ago, in his Confessions, Saint Augustine (354-430) posed the obvious question in biblical terms: What was God doing before he created the Universe?

ISAAC NEWTON: COLOR SPECTRUM and the CORPUSCULAR THEORY of LIGHT

In the graphic below, light enters the prism from the top right, and is refracted by the glass. The violet is bent more than the yellow and red, so the colors separate.

In the 1660s, Newton began experimenting with his “celebrated phenomenon of colors.” At the time, people thought that color was a mixture of light and darkness, and that prisms colored light. Hooke was a proponent of this theory of color, and had a scale that went from brilliant red, which was pure white light with the least amount of darkness added, to dull blue, the last step before black, which was the complete extinction of light by darkness. Newton believed this theory was false.

Newton set up a prism near a window at his boyhood home in Woolsthorpe, England ( site of the famous apple tree), and projected a beautiful spectrum 22 feet onto the far wall. Further, to prove that the prism was not coloring the light, he refracted the spectral light back together, producing white light. Incidentally, he was at home because all the students at Cambridge University where he was a student were sent home because of an epidemic of the bubonic plague. In 1665, it was a version of “ social distancing .” BTW, Newton did his best work working from home. On a personal note, my wife and I once visited the hometown of William Shakespeare (1564-1616), Stratford-upon-Avon. Access to Anne Hathaway’s cottage was severely hampered by rows of tour buses, cars, and floods of pedestrians. Later, we drove to Woolsthorpe by Colsterworth (took about an hour on country roads) to visit Newton’s home, now a museum. When we arrived the museum was closed, and had been for some time. I asked a local gentleman “why,” and he responded “because no one ever came.” The diagram below is from Sir Isaac Newton’s crucial experiment, 1666-72. A ray of light is divided into its constituent colors by the first prism (left), and the resulting bundle of colored rays is reconstituted into white light by the second prism (right).

Artists were fascinated by Newton’s clear demonstration that light alone was responsible for color. His most useful idea for artists was his conceptual arrangement of colors around the circumference of a circle, which allowed the painters’ primaries (red, yellow, blue) to be arranged opposite their complementary colors (e.g. red opposite green), as a way of denoting that each complementary color would enhance the other’s effect through optical contrast.

In  optics , the corpuscular theory of light, arguably set forward by  Descartes  in 1637, states that  light  is made up of small discrete particles called " corpuscles " (little particles) which travel in a straight line with a  finite velocity  and possesses momentum (mass x velocity). Isaac Newton  was a pioneer of this theory, an elaboration of his view of reality as interactions of material points through forces. The following passage gives Albert Einstein 's description of Newton's conception of physical reality:

“[Newton's] physical reality is characterized by concepts of  space ,  time , the  material point  and  force  (interaction between  material points ). Physical events are to be thought of as movements according to law of  material points  in space. The  material point  is the only representative of reality in so far as it is subject to change.”  This early conception of the  particle theory of light  was an early forerunner to the modern understanding of the  photon . See next Lecture 14.

CHRISTIAAN HUYGENS: WAVE THEORY of LIGHT The corpuscular theory cannot explain  refraction ,  diffraction  and  interference , which require an understanding of the  wave theory of light  of  Christiaan Huygens . refraction: the change in direction of a wave passing from one medium to another or from a gradual change in the medium. Refraction of light is the most commonly observed phenomenon, but other waves such as sound waves and water waves also show refraction. diffraction: Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit. It is defined as the bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. interference: interference is a phenomenon in which two waves superimpose to form a resultant wave of greater (constructive interference), lower (destructive interference), or the same amplitude.  To explain the origin of color,  Robert Hooke  (1635–1703) developed a "pulse theory" and compared the spreading of light to that of waves in water in his 1665 Micrographia. In 1672 Hooke suggested that light's vibrations could be  perpendicular  to the direction of propagation.  Christiaan Huygens  (1629–1695) worked out a mathematical wave theory of light in 1678, and published it in his  Treatise on light  in 1690. He proposed that light was emitted in all directions as a series of waves in a hypothetical medium called the  Luminiferous ether (invisible to the senses, no “weight,” color, odor or taste) and that, as waves are not affected by gravity, he assumed that they slowed down upon entering a denser medium. That light waves could interfere with each other like sound waves was first noted around 1800 by  Thomas Young , a physician. Young demonstrated by means of a  diffraction experiment  that light behaved as waves. He also proposed that different colors were caused by different  wavelengths  of light, and explained color vision in terms of three color receptors in the eye.

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Thomas Young 's sketch of a  double-slit experiment  showing  diffraction . Young's experiments supported the theory that light consists of waves.

Coincidentally, Young’s report of the double-slit experiment appeared in the Proceedings of the Royal Society of London in the same year (1800) as Volta’s paper on his invention of the battery. In earlier lectures in the course, examples of the atomic structure of inorganic, organic and biochemical entities were shown [ e.g., NaCl (Lecture 1) , diamond (Lecture 5), COVID-19 (Lecture 12) ]. Did you ever ask yourself (or me) how those crystal structures were determined? Thank you, Thomas Young. Let’s get to the bottom of this. ELECTROMAGNETIC WAVES and POLARIZATION One speaks of the wave nature of light, but we have not specified what it is that oscillates or undulates. It is easy to visualize waves in a fluid medium such as water, where we can see the motion of the water as the wave passes by. wavelength (λ) = the distance, measured in the direction of propagation of a wave, from any given point to the next point in the same phase.

frequency (ν) = the number of oscillations (or cycles) of a wave per unit of time

We now know that electromagnetic waves are very different from water waves or sound waves. NO MEDIUM is REQUIRED. Light waves can travel through a total vacuum, unlike other waves with which we are familiar. Recognizing the significance of this difference, established experimentally by Michaelson and Morley in 1887 at what is now Case Western University in Cleveland, is one of the two cornerstones on which Einstein developed his Theory of Special Relativity, published in 1905. We shall pursue this in Lecture 16 on nuclear chemistry, nuclear medicine and radiography. The Nobel Prize in Physics 1907 was awarded to Albert Abraham Michelson "for his optical precision instruments and the spectroscopic and metrological investigations carried out with their aid." Importantly, when watching water waves lap against the shore on Lake Michigan, it appears that water is “moving” toward shore. Not so. The water in the lake is not moving. What is being transported is energy. In the classical theory of waves, the energy is given by E = ½ k A2 ( k = a constant, A = amplitude ) In the quantum theory of light (to be presented in the next Lecture 14), the energy is given by E = h ν ( h = a constant, ν = frequency) Rather than having physical motions, electromagnetic (light) waves consist of electric and magnetic fields that oscillate back and forth at right angles to the direction of motion. The electric and magnetic fields lie in planes that are also perpendicular to each other, so that the electric field flips back and forth in one plane, with the magnetic field doing so in the plane that lies perpendicular to it, and both are traverse to the direction of travel.

Electromagnetic (light) waves can be imagined as a self-propagating, transverse, oscillating wave of electric and magnetic fields. The above diagram shows a plane, linearly polarized wave propagating from left to right. The electric field is in a vertical plane and the magnetic field in a horizontal plane.

A photon (a quantum or Newtonian “corpuscle” of light, to be introduced in Lecture 14) can be viewed as a packet of electromagnetic energy consisting of alternating fields, which, by virtue of their wavelength and frequency, provide the photon with its wave properties.

Summarizing the above discussion, light is energy that radiates, or travels, in waves. Light is a wave of vibrating electric and magnetic fields. Visible light is one small part of a larger range of vibrating electromagnetic fields. This range is called the electromagnetic spectrum. By contrast, sound is a wave of vibrating air. The energy of the radiation depends on its wavelength and frequency. As noted above, wavelength is the distance between the tops (crests) [ or bottoms (troughs) ] of the waves. And, frequency is the number of waves that pass by each second. The longer the wavelength of the light, the lower the frequency, and the less energy it contains.

Recalling that Velocity (v) = distance /time = λ ν

electromagnetic waves travel through space at 299,792 km/sec (186,282 miles/sec). This is called the velocity (speed) of light. In Astronomy, the distance a beam of light travels in one year is called a light year.  The light emitted at the first moment of the Big Bang (see the photo taken with the Hubble telescope at the top of this Lecture) is 46.1 billion (46.1x109) light-years distant. Light from most sources (such as stars) consists of vast numbers of photons, each with a specific, constant orientation of its electric and magnetic planes. Ordinarily the orientation of the various photons is random, but under some circumstances it is not. If light passes through or reflects from a medium with a certain orientation, what is left is light in which all the photons are aligned. That is, the planes of the electric and magnetic fields of the photons are parallel. The light is said to be polarized.

Many of the sunglasses sold today consist of polarizing filters, and they have the effect of screening out light in a given orientation. In Astronomy, polarization occurs naturally, sometimes because a natural filtering occurs, as in the interstellar medium, and in other cases because the source of the light emits radiation with a preferred orientation.

Next, we need to understand the following phenomena. It happens that one of the things about waves is the following:  If they encounter an obstacle which is smaller than their wavelength, the wave just kind of wraps around the object and keeps on going.  This is why you can hide behind a big tree trunk and the voices of people on the other side of the tree will still reach you (because the long waves of the sound wrap around the tree). BUT, light from their flash light won't (because the short waves of the light can't wrap around such a big object). Sound and light act similarly when they reach a corner of a house - the sound waves are long and kind of wrap around the corner and then spread out from there, whereas the very short light waves can't bend around the corner. As you might have guessed already, this means that light does wrap around very small things (things smaller than a millionth of a foot), and does bend a tiny bit around corners, but this bending is so small it is impossible to see with your eye. See, however, the following discussion.

X-rays are light waves of very short wavelength and high energy discovered by Wilhelm Röntgen (1845-1923) who, on November 8, 1895, produced and detected  electromagnetic radiation  in a  wavelength  range known today as  x-rays . 

Roentgen’s experiment involved studying the radiation emitted outside a cathode ray tube (see description in the following Lecture 14). He also used as sensors a series of compounds that were sensitive to light of high frequencies, the principal one of which was the barium salt of the platinocyanide ion. The ion exists in two isometric forms:

Barium platinocyanide, Ba[Pt(CN)4] is a phosphor and a scintillator. It fluoresces in the presence of light of high frequencies (ν)

One of the things Röntgen discovered about x-rays was their incredible penetrating power. They could pass through paper, glass, wood and human flesh (but not lead). He asked his wife Bertha to put her hand between the source of x-rays and a photographic plate. The result was one of the most famous of all photographs.

Within months x-rays were used in medicine worldwide to diagnose fractures. X-rays remain today one of the most important diagnostic tools in medicine. The first Nobel Prize in Physics was awarded in 1901 to Wilhelm Conrad Röntgen "in recognition of the extraordinary services he has rendered by the discovery of the remarkable rays subsequently named after him." The discovery of x-rays launched a new era in experimental physics and chemistry which led to the quantum theory of atoms and molecules. See the next Lecture 14. Max von Laue (1879 – 1960) discovered the diffraction of x-rays by crystals. The idea came to him that estimates of the size of atoms, and their spacing in a crystal, were about the same as the wavelength of Röntgen’s x-rays. He suggested that the much shorter electromagnetic rays, which x-rays were supposed to be, would cause in a crystal some kind of diffraction or interference phenomena. Knipping, an experimentalist, tested out the idea and, after some failures, succeeded in proving it to be correct. Von Laue worked out the mathematical formulation and the discovery was published in 1912. For this, he was awarded the Nobel Prize in 1914. It established the fact that x-rays are electromagnetic in nature and it opened the way to the later theoretical and experimental work of  Sir William and Sir Lawrence Bragg . Sir William Lawrence Bragg 1890 – 1971) was an Austalian-born British physicist, discoverer in 1912 of  Bragg's law of x-ray diffraction , which is basic for the determination of  crystal structure . He was joint recipient (with his father,  William Henry Bragg ) of the  Nobel Prize in Physics  in 1915.

Bragg was the director of the  Cavendish Laboratory , Cambridge, when the discovery of the structure of DNA (the double helix, see Lecture 6 ), based on the x-ray diffraction images of DNA by Rosalind Franklin, was reported by James D. Watson and  Francis Crick  in February 1953. Watson, Crick and Maurice Wilkins received the Nobel Prize in Physiology or Medicine in 1962.   Also at the Cavendish Laboratory was Max Perutz (1914-2002), an Austrian-born British  molecular biologist , who shared the 1962  Nobel Prize for Chemistry  with  John Kendrew , for their studies of the crystal structures of  hemoglobin  and  myoglobin (See Lecture 6 )  Another at Cambridge was Aaron Klug, a  Lithuanian -born, South African-educated, British  biophysicist . He solved the crystal structures of macromolecules such as viruses (see Lecture 12) working with Rosalind Franklin. He found the rules of the geometrical form of poliovirus and other spherical viruses. His invention of electron tomography, in which a 3D image of a virus is obtained from many electron micrographs, led to the Nobel Prize in Chemistry in 1982. Rosalind Franklin died at the age of 37 from ovarian cancer. Watson, among many others, suggested that Franklin should have been awarded a  Nobel Prize in Chemistry , along with Wilkins, but, although there was not yet a rule against posthumous awards, the Nobel Committee generally did not make posthumous nominations. The experimental technique of x-ray crystallography is the common thread which links together much of what has been highlighted in earlier lectures, and provides a good review of some of the most important discoveries in inorganic chemistry, biochemistry and medicinal chemistry.

Officially, X-ray crystallography is defined as the experimental science determining the atomic and molecular structure of a  crystal , in which the crystalline structure causes a beam of incident x-rays to  diffract  into many specific directions. By measuring the angles and intensities of these diffracted beams, a  crystallographer  can produce a three-dimensional picture of the density of  electrons  within the crystal. From this  electron density , the mean positions of the atoms in the crystal can be determined, as well as their  chemical bonds and their crystallographic disorder .

COLORS of LIGHT Visible light is the part of the electromagnetic spectrum that our eyes can see. Light from the sun or a light bulb may look white, but it is actually a combination of many colors. We can see the different colors of the spectrum by splitting the light with a prism. The spectrum is also visible when you see a rainbow in the sky. The colors blend continuously into one another. At one end of the spectrum are the reds and oranges. These gradually shade into yellow, green, blue, indigo and violet. The colors have different wavelengths, frequencies, and energies. Violet has the shortest wavelength in the visible spectrum. That means it has the highest frequency and energy. Red has the longest wavelength, and lowest frequency and energy.

PERCEPTION of COLOR

When a sample absorbs visible light, an object will selectively absorb specific wavelengths of light from the complete spectrum of possibilities. The color we perceive is the sum of the remaining colors that are reflected or transmitted by an object and strike our eyes. An object which reflects light is said to be opaque. An object which transmits all light is said to be transparent. An object which absorbs all wavelengths of visible light appears black (because no light reaches our eyes). An object which absorbs no visible light appears white or colorless. In case you’re wondering, …. What is a rainbow? A rainbow occurs when light hits the water drops in the atmosphere at a certain angle. It is an atmospheric phenomenon that is formed through optical processes such as refraction, dispersion, internal reflection and secondary refraction. If the light source is the sun, then the rainbow will be colorful and bright. Why is water blue? Water does absorb a tiny amount of light, but in such small amounts, we don’t notice it. Water actually absorbs light that has a reddish color. When there’s a lot of water, like Lake Michigan, we notice that red light from the Sun has actually been absorbed. So, what color do we see when reddish light is gone? Blue!

Blue is the color absorbed the least by water, so it’s the one we see the most. Blue light also scatters more than other light. That’s the reason why the sky is blue. That scattering affects the color of water too.

It just takes a big enough sample of water to notice these things. That’s why you won’t see color in the glass you’re drinking.

Why is the grass green? The blades of grass and the leaves of trees contain the molecule chlorophyll (see Lecture 5 ) a molecule able to absorb in the red (and blue) regions of the spectrum. When white sunlight is reflected from leaves, what we see is the light not absorbed by the plant. The color vegetation appears to our senses, the color which is not absorbed, is green. Red and green are complementary colors.

Why is the color of an orange, orange ? Orange and blue are complementary colors. The removal of blue light from white light makes the light look orange. [Conversely, the removal of orange light from white light makes the light look blue.] If light of all colors except blue strikes our eyes, we perceive an orange color. If an orange absorbs all light except orange, in the reflected light, the fruit appears orange. In his book “Origins,” Richard Leaky notes that the perception of color in primates was a key factor in human evolution. He displays a color photograph of an orange tree, leaves in green and oranges in orange, and compares this with a black-and-white photo. If you’re looking for fruit to eat, the advantage of color vision is obvious. Why are minerals colored?

Color in minerals is caused by the absorption, or lack of absorption, of various wavelengths of light. The color of light is determined by its wavelength. When pure white light (containing all wavelengths of visible light) enters a crystal, some of the wavelengths might be absorbed while other wavelengths may be emitted. If this happens, then the light that leaves the crystal will no longer be white but will have some color. Small amounts of impurities can also change the color of crystals. Recall the Hope Diamond (Lecture 5). My colleague, Harry Gray at Caltech, pointed out the remarkable fact that only small differences in Cr(III)O6 ligand fields make emeralds green and rubies red !

COLOR and the HUMAN BRAIN Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths (or frequencies ) of the light they reflect, emit, or transmit. Colors can be measured and quantified in various ways. A human's perception of colors is a subjective process whereby the brain responds to the stimuli that are produced when incoming light reacts with the several types of photoreceptors in the eye . Rods and Cones are the photoreceptors found in the retina.  Rods have rod-like structure and provide twilight vision. Cones are of the cone shape, fewer in number and provide vision in the day or bright light.  Rods are found around the boundary of the retina, whereas cones are there in the center of the retina. Color vision relies on a brain perception mechanism that treats light with different wavelengths as different visual stimuli (e.g., colors). Usual color insensitive photoreceptors (the rods in human eyes) only react to the presence or absence of light and do not distinguish between specific wavelengths. NOTE: Colors are not “real.” They are “synthesized” by our brain to distinguish light with different wavelengths. While rods give us the ability to detect the presence and intensity of light (and thus allow our brain to construct the picture of the world around us), specific detection of different wavelengths through independent channels gives our view of the world additional high resolution. For instance, red and green colors look like near identical shades of grey in black and white photos.

An animal with black and white vision alone won’t be able to make a distinction between, let’s say, a green and red apple, and won’t know which one tastes better before trying them both based on color. Evolutionary biologists believe that human ancestors developed color vision to facilitate the identification of ripe fruits, which would obviously provide a selective advantage in the competitive natural world. The same conclusion had been reached by the anthropologist, Richard Leaky.

Why certain wavelengths are paired with certain colors remains a mystery. As mentioned above, technically, color is an illusion created by our brain. Therefore, it is not clear if other animals see colors the same way we see them. It is likely that, due to shared evolutionary history, other vertebrates see the world colored similarly to how we see it. Color vision is quite common across the vast animal kingdom: insects, arachnids, and cephalopods are able to distinguish colors.

What kind of colors do these animals see? As was first proposed by Thomas Young, human color vision relies on three photoreceptors that detect primary colors: red, green, and blue. However, some people lack red photoreceptors (they are “bichromates”) or have an additional photoreceptor that detects somewhere between red and green colors (“tetrachromates”). Having only three photoreceptors doesn’t limit our ability to distinguish other colors.

Each photoreceptor can absorb a rather broad range of wavelengths of light. To distinguish a specific color, the brain compares and quantitatively analyses the data from all three photoreceptors. Our brain does this remarkably successfully. Some research indicates that we can distinguish colors that correspond to wavelength differences of just one nanometer (10-9 meters)

This scheme works in largely the same way in most higher vertebrate animals that have color vision. Although the ability to distinguish between specific shades varies significantly between the species, with humans having one of the best color distinguishing abilities.

However, invertebrates that have developed color vision (and vision in general) completely independently from us demonstrate remarkably different approaches to color detection and processing. These animals can have an exceptionally large number of color receptors. The mantis shrimp, for instance, has 12 different types of photoreceptors. The common bluebottle butterfly has even more, 15 receptors.

Does it mean that these animals can see additional colors unimaginable to us? Perhaps yes. Some of their photoreceptors operate in a rather narrow region of light spectrum. For instance, they can have 4-5 photoreceptors sensitive in the green region of the visual spectrum. This means that for these animals the different shades of green may appear as different as blue and red colors appear to our eyes! Again, the evolutionary advantages of such adaptations are obvious for an animal living among the trees and grasses where most objects, as we see them, are colored in various shades of green.

Researchers tried to test if a more complicated set of visual receptors provide any advantages for animals when it comes to the distinguishing between main colors. The findings show that this is not necessarily the case, at least not for the mantis shrimp. The mantis shrimp has an impressive array of receptors detecting light in a much broader part of the electromagnetic spectrum compared to humans. They determine the colors fast. This is important for practical purposes, as mantis shrimps are predators. A large number of photoreceptors allows for their quick activation at specific wavelengths of light and thus communicate directly to the brain what specific wavelength was detected. In comparison, humans have to assess and quantify the signals from all three photoreceptors to decide on a specific color. This requires more time and energy.

Apart from employing a different number of photoreceptors to sense light of specific wavelengths, some animals can detect light that we humans are completely unable to see. For example, many birds and insects can see in the UV part of the spectrum. Bumblebees, for instance, have three photoreceptors absorbing in the UV, blue, and green regions of the spectrum. This makes them trichromates, like humans, but with the spectral sensitivity shifted to the blue end of the spectrum. The ability to detect UV light explains why some flowers have patterns visible only in this part of the spectrum. These patterns attract pollinating insects, which have an ability to see in this spectral region.

A number of animals can detect infrared light (the long wavelength radiation) emitted by heated objects and bodies. This ability significantly facilitates hunting for snakes that are usually looking for small warm-blooded prey. Seeing them through IR detecting receptors is a great tool to detect slow-moving reptiles. The photoreceptors sensitive to IR radiation in snakes are located not in their eye but in “pit organs” located between the eyes and nostrils. The result is still the same: Snakes can color objects according to their surface temperature.

Wavelength and hue detection

As noted earlier, Isaac Newton discovered that white light splits into its component colors when passed through a prism, but that if those bands of colored light pass through another and rejoin, they make a white beam. The characteristic colors are, from low to high frequency: red, orange, yellow, green, cyan, blue, violet. Sufficient differences in frequency give rise to a difference in perceived hue . The just noticeable difference in wavelength varies from about 1 nm in the blue-green and yellow wavelengths, to 10 nm and more in the red and blue. Though the eye can distinguish up to a few hundred hues, when those pure spectral colors are mixed together or diluted with white light, the number of distinguishable chromaticities can be quite high.

In very low light levels, vision is scotopic . Light is detected by rod cells of the retina. Rods are the principal players. In brighter light, such as daylight, vision is photopic . Light is detected by cone cells which are responsible for color vision. Cones are sensitive to a range of wavelengths, but are most sensitive to wavelengths near 555 nm (or, 5550 Å). Between these regions, mesopic vision comes into play and both rods and cones provide signals to the retinal ganglion cells . The shift in color perception from dim light to daylight gives rise to differences known as the Purkinje effect .

The perception of “white” is formed by the entire spectrum of visible light, or by mixing colors of just a few wavelengths, such as red, green, and blue, or by mixing just a pair of complementary colors such as blue and yellow.

PHUSIOLOGY of COLOR PERCEPTION Perception of color begins with specialized retinal cells containing pigments with different spectral sensitivities , known as cone cells . In humans, there are three types of cones sensitive to three difference spectra, resulting in trichromatic color vision .

The cones are conventionally labeled according to the ordering of the wavelengths of the peaks of their spectral sensitivities : short (S), medium (M), and long (L) cone types. These three types do not correspond well to particular colors as we know them. Rather, the perception of color is achieved by a complex process that starts with the differential output of these cells in the retina and it will be finalized in the visual cortex and associative areas of the brain.

For example, while the L cones have been referred to simply as red receptors, their peak sensitivity is in the greenish-yellow region of the spectrum. Similarly, the S- and M-cones do not directly correspond to blue and green , although they are often depicted as such. The peak response of human cone cells varies, even among individuals with normal color vision. In non-human species this polymorphic variation is even greater, and it may well be adaptive. Normalized response spectra of human cones, S, M, and L types, to monochromatic spectral stimuli, with wavelength given in nanometers.

The same figures as above represented here as a single curve in three (normalized cone response) dimensions.

Relative brightness sensitivity of the human visual system as a function of wavelength.

Two complementary theories of color vision are the trichromatic theory and the opponent process theory. The trichromatic theory, or Young–Helmholtz theory , proposed in the 19th century by Thomas Young and Hermann von Helmholtz , states that the retina's three types of cones are preferentially sensitive to blue, green, and red. Ewald Hering proposed the opponent process theory in 1872. It states that the visual system interprets color in an antagonistic way: red vs. green, blue vs. yellow, black vs. white. We now know both theories to be correct, describing different stages in visual physiology.

Cone cells in the human eye

A range of wavelengths of light stimulates each of these receptor types to varying degrees. Yellowish-green light, for example, stimulates both L and M cones equally strongly, but only stimulates S-cones weakly. Red light, on the other hand, stimulates L cones much more than M cones, and S cones hardly at all. Blue-green light stimulates M cones more than L cones, and S cones a bit more strongly, and is also the peak stimulant for rod cells; and blue light stimulates S cones more strongly than red or green light, but L and M cones more weakly. The brain combines the information from each type of receptor to give rise to different perceptions of different wavelengths of light.

The opsins (photopigments) present in the L and M cones are encoded on the X chromosome ; defective encoding of these leads to the two most common forms of color blindness . The OPN1LW gene, which codes for the opsin present in the L cones, is highly polymorphic (a recent study found 85 variants in a sample of 236 men).

A very small percentage of women may have an extra type of color receptor because they have different alleles for the gene for the L opsin on each X chromosome. X chromosome inactivation means that only one opsin is expressed in each cone cell, and some women may therefore show a degree of tetrachromatic color vision. Variations in OPN1MW, which codes the opsin expressed in M cones, appear to be rare, and the observed variants have no effect on spectral sensitivity .

Now, for the brain per se.

Visual pathways in the human brain. The ventral stream (purple) is important in color recognition. The dorsal stream (green) is also shown. They originate from a common source in the visual cortex .

Color processing begins at a very early level in the visual system (even within the retina) through initial color opponent mechanisms. Both the Young-Helmholtz trichromatic theory, and Hering's opponent process theory are therefore correct, but trichromacy arises at the level of the receptors, and opponent processes arise at the level of retinal ganglion cells and beyond.

In Hering's theory opponent mechanisms refer to the opposing color effect of red–green, blue–yellow, and light–dark. However, in the visual system, it is the activity of the different receptor types that are opposed. Some midget retinal ganglion cells oppose L and M cone activity, which corresponds loosely to red–green, but actually runs along an axis from blue-green to magenta. Small bistratified retinal ganglion cells oppose input from the S cones to input from the L and M cones. This is often thought to correspond to blue–yellow ,opponency, but actually runs along a color axis from lime green to violet.

Visual information is then sent to the brain from retinal ganglion cells via the optic nerve to the optic chiasma , a point where the two optic nerves meet and information from the temporal (contralateral) visual field crosses to the other side of the brain. After the optic chiasma (the X-shaped structure formed at the point below the brain where the two optic nerves cross over each other), the visual tracts are referred to as the optic tracts , which enter the thalamus to synapse at the lateral geniculate nucleus (LGN).

The LGN is divided into laminae (zones), of which there three types: the M-laminae, consisting primarily of M-cells, the P-laminae, consisting primarily of P-cells, and the koniocellular laminae. M- and P-cells received relatively balanced input from both L- and M-cones throughout most of the retina, although this seems to not be the case at the fovea, with midget cells synapsing in the P-laminae. The koniocellular laminae receive axons from the small bistratified ganglion cells. After synapsing at the LGN, the visual tract continues on back to the primary visual cortex (V1) located at the back of the brain within the occipital lobe . Within V1 there is a distinct band (striation). This is also referred to as "striate cortex", with other cortical visual regions referred to collectively as "extrastriate cortex". It is at this stage that color processing becomes much more complicated.

In V1 the simple three-color segregation begins to break down. Many cells in V1 respond to some parts of the spectrum better than others, but this "color tuning" is often different depending on the adaptation state of the visual system. A given cell that might respond best to long wavelength light if the light is relatively bright might then become responsive to all wavelengths if the stimulus is relatively dim. Because the color tuning of these cells is not stable, some believe that a different, relatively small, population of neurons in V1 is responsible for color vision. These specialized "color cells" often have receptive fields that can compute local cone ratios. Such "double-opponent" cells were initially described in the goldfish retina by Nigel Daw. Their existence in primates was suggested by David H. Hubel and Torsten Wiesel and subsequently proven by Bevil Conway . As Margaret Livingstone and David Hubel showed, double opponent cells are clustered within localized regions of V1 called blobs , and are thought to come in two flavors, red–green and blue–yellow. Red–green cells compare the relative amounts of red–green in one part of a scene with the amount of red–green in an adjacent part of the scene, responding best to local color contrast (red next to green). Modeling studies have shown that double-opponent cells are ideal candidates for the neural machinery of color constancy explained by Edwin H. Land in his retinex theory. The Nobel Prize in Physiology or Medicine 1981 was divided, one half awarded to Roger W. Sperry "for his discoveries concerning the functional specialization of the cerebral hemispheres", the other half jointly to David H. Hubel and Torsten N. Wiesel "for their discoveries concerning information processing in the visual system."

From the V1 blobs, color information is sent to cells in the second visual area, V2. The cells in V2 that are most strongly color tuned are clustered in the "thin stripes" that, like the blobs in V1, stain for the enzyme cytochrome oxidase (separating the thin stripes are interstripes and thick stripes, which seem to be associated with other visual information like motion and high-resolution form). Neurons in V2 then synapse onto cells in the extended V4. This area includes not only V4, but two other areas in the posterior inferior temporal cortex, anterior to area V3, the dorsal posterior inferior temporal cortex, and posterior TEO. Color processing in the extended V4 occurs in millimeter-sized color modules ( globs ). This is the first part of the brain in which color is processed in terms of the full range of hues found in color space .

Anatomical studies have shown that neurons in extended V4 provide input to the inferior temporal lobe . The "IT" cortex is thought to integrate color information with shape and form, although it has been difficult to define appropriate criteria for this claim. Despite this murkiness, it has been useful to characterize this pathway (V1 > V2 > V4 > IT) as the ventral stream or the "what pathway", and distinguished from the dorsal stream ("where pathway") that is thought to analyze motion.