The retina also is home to another type of light-sensitive cell. These melanopsin ganglion (MEH-lah-NOP-sin GANG-lee-un) cells don’t send signals to the occipital cortex. Instead, they report the presence of light to the olivary pretectal nucleus (OH-liv-airy Pree-TEK-tahl NEW-klee-us). This is a tiny spot in the middle of the base of the brain. The signals that melanopsin ganglion cells send here help regulate the body’s master biological clock. They also send signals that control the size of the pupil (which controls how much light gets into the eye in the first place).

Red, green, blue, black and white. These five colors end up making every single color that we see. Cone cells are especially concentrated in the fovea, and work only in bright light. At night, you need your rods.

The tissue there, known as the retina, contains millions of light-sensitive cells. They are especially concentrated in an area called the fovea (FOH-vee-ah). This densely packed set of cells gives us the clearest picture of our world. When the eye focuses on an object, it directs the light bouncing off the object directly onto the fovea to get the best image. In fact, when the eye focuses on something, that’s called foveating (FOH-vee-ayt-ing).

cell: The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

How do your eyes work? It’s far more than just forming a tiny picture in your eye. There’s also color and motion. It takes many cells — and finally the brain — to make sense of it all.

cones: (in biology) A type of eye cell that is part of the retina inside the back of the eye. These cells can sense red, green or blue light. Recent research has uncovered evidence that many can sense white light — but only white light.

nerve: A long, delicate fiber that transmits signals across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold or pain.

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How do we see with our eyes

pigment: A material, like the natural colorings in skin, that alter the light reflected off of an object or transmitted through it. The overall color of a pigment typically depends on which wavelengths of visible light it absorbs and which ones it reflects. For example, a red pigment tends to reflect red wavelengths of light very well and typically absorbs other colors. Pigment also is the term for chemicals that manufacturers use to tint paint.

cornea: The transparent front section of the eye. The shape of the cornea allows our eyes to bring objects at many distances into focus.

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receptor: (in biology) A molecule in cells that serves as a docking station for another molecule. That second molecule can turn on some special activity by the cell.

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molecule: An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

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Each rod or cone cell at the back of the eye has a stack of discs inside, The discs contain a pigment molecule. It’s bound to a protein called an opsin. Rods and cones each have a different opsin.

Orderinwhichlightpasses throughthe eye

photopsin: A pigment molecule bound to the light-sensing protein opsin. Photopsins are found in the cone cells of the eye and come in three types — red, blue and green. They also can sense white and black.

Refraction oflight in the eyeoccurs at

Electromagnetic waves are transverse, and can be polarised. The polarisation of light is used in many applications.

biological clock: A mechanism present in all life forms that controls when various functions such as metabolic signals, sleep cycles or photosynthesis should occur.

In September 2016, a vision researcher at the University of Washington in Seattle discovered some cones also sense white light. But only white light. That was a big surprise, Ramkumar Sabesan said at the time.

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lens: (in biology) A transparent part of the eye behind the colored iris that focuses incoming light onto the light-absorbing membrane at the back of the eyeball. (in physics) A transparent material that can either focus or spread out parallel rays of light as they pass through it..

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HowDoes lightenterthe eye

tissue: Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.

The retina’s rod cells aren’t part of the cone coloring system. They work when light levels are low. Instead of photopsins, rods have a different pigment-protein pair: rhodopsin (Roh-DOP-sin). Rods produce images only in shades of grey. But they are much more sensitive to light than cones are. They are so sensitive that a rod cell can detect a single photon of light — the smallest possible particle.

focus: (In vision, verb, "to focus") The action a person's eyes take to adapt to light and distance, enabling them to see objects clearly. (in behavior) To look or concentrate intently on some particular point or thing.

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In fact, he and his colleagues found, so-called red and green cone cells each come in two types. One transmits white light, the other relays color. Especially surprising, most of these cones are the white type. Out of 167 red cones tested, 119 signaled white. Of 98 green cones tested, 77 reported white light. (The team didn’t test white sensitivity among the retina’s few blue cones.)

Cones have a pigment-protein pair called photopsin (Foh-TOP-sin). It comes in three different types, and each cone has just one type. They come in red, green or blue — the colors that each cone type is best at absorbing. Cones respond to light that has passed through the lens and onto the fovea. As each cone absorbs its color of light, it produces an electrical signal. These signals travel to the brain, filling our worlds with color.

As light enters our eyes, it first heads through a tough outer tissue called the cornea. This protects the delicate inner eye from everything the world might throw at it. Light passes right through the cornea and into a transparent, flexible tissue called the lens. This lens focuses the light, sending it through the liquid-filled globe of the eyeball to the back interior wall of the eye.

Light signals sent to this master body clock tell you when to be sleepy and when to be alert. But not just any light will do. This clock can distinguish between different colors of light. Blue works best to stimulate the body clock. Sunlight is an excellent source of blue light. Although it looks white, sunlight actually is a mix of many colors, including blue. This might explain why stepping outside on a bright sunny day helps clear the fog from your head.

nucleus: Plural is nuclei. (in biology) A dense structure present in many cells. Typically a single rounded structure encased within a membrane, the nucleus contains the genetic information. (in astronomy) The rocky body of a comet, sometimes carrying a jacket of ice or frozen gases. (in physics) The central core of an atom, containing most of its mass.

When they detect certain wavelengths of visible light, the photoreceptors trigger electrical signals. Rods and cones will send these signals through nerves that reach into the brain. They head for the occipital (Awe-SIP-ih-tal) cortex, right up against the back of the skull. There, the brain interprets these signals to make sense of what we’re looking at.

chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

Howdoes the eyework simple explanation

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pupil: (in biology) The dark center of an eye. The pupil is actually a hole in the eye that allows light to pass through and hit the retina, the part of our eye that is sensitive to light.

wavelength: The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red).

Journal:​ ​​ R. Sabesan et al. The elementary representation of spatial and color vision in the human retina. Science Advances. September 14, 2016, p. e1600797. doi: 10.1126/sciadv.1600797.

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The light-sensing cells on the retina are known as photoreceptors. Two important types are rods and cones. Each human retina (and you have two, one in each eye) contains 125 million rods and about 6 million cones. This is 70 percent of all the sensory receptors in your entire body — for touch, taste smell, hearing and sight all put together. That’s how important vision is to us.

In the dark, we rely on our rods. But light inactivates these cells. It stimulates them so much that they become unresponsive. That’s all right, cones are there to take over. They require much more light to function. So we rely on cones in the light.

body clock: (also known as biological clock ) A mechanism present in all life forms that controls when various functions such as metabolic signals, sleep cycles or photosynthesis should occur.

retina: A layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses that travel along the optic nerve to the brain, where a visual image is formed.

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olivary pretectal nucleus: A small group of cells in the center of the brain, at the base. These cells control how big the pupils of your eyes are, and how much light they let in. They also help tell the body what time it is.

10 uses of eyes

melanopsin ganglion cells: Light-sensitive cells in the eye that send signals to the olivary pretectal nucleus. The signals regulate the size of the eye’s pupil, and help control the body clock.

White-sensing cells also detect black (which is the absence of white). The data they relay create a sharp black-and-white picture of someone’s surroundings. These provide a crisp edge to visual details. Red- and green-signaling cells fill in the lines with blurrier chunks of color. The process, says Sabesan, works much like filling in a coloring book or adding color to a black-and-white film.

protein: A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

Website: Rods and cones of the human eye. An Ask a Biologist report from Arizona State University’s School of Life Sciences.

fovea: A small depression at the center back of the eye's retina. Color-sensing cone cells are especially concentrated here. The fovea also is the site of peak visual acuity.

rods: (in biology) A type of eye cell that is part of the retina inside the back of the eye. These cells are rod shaped and sensitive to light. Although more sensitive to light than cone cells are, rods can not tell what color something is.

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rhodopsin: A combination of a pigment molecule and the light-sensing protein opsin. Rhodopsins are found in the red cells of the eye. They are extremely sensitive to light, but cannot sense color.

Journal: G. Campbell and A. R. Lieberman. The olivary pretectal nucleus: experimental anatomical studies in the rat. Proceedings of the Royal Society: Biological Sciences. October 14, 1985. Vol. 310, p. 573–609. doi: 10.1098/rstb.1985.0132.