![Reflection and refraction of light Reflection - Law of reflection: The angle of reflection ’ equals to the angle of incidence . The ray reflected travels in the plane of incidence. Refraction: When light passes from one medium into another, the beam changes direction at the boundary between the two media. This property of optical media is characterised by index of refraction n = c/v [ dimensionless ] n – index of refraction of respective medium c – speed of light in vacuum v – speed of light in the respective medium index of refraction of vacuum is 1](https://image.slidesharecdn.com/visionbf-1h-fin-110914112002-phpapp02/85/Visionbf-1h-fin-6-320.jpg)
![Lens–make r’ s equation f - focal distance (length) [m] n 2 - index of refraction of the lens n 1 - index of refraction of the medium r 1 , r 2 - radii of curvature of the lens](https://image.slidesharecdn.com/visionbf-1h-fin-110914112002-phpapp02/85/Visionbf-1h-fin-8-320.jpg)
![Common principles of optical imaging Real image (can be projected): convergence of rays Virtual image (cannot be projected): divergence of ray Principal axis – optical axis of centred system of optical boundaries Principal focus is a point where rays parallel to the principal axis intersect after refraction by the lens or reflection by the curved mirror - front ( object ) focus and back (image) focus Focal distance (length) f [m] is the distance of focus from the centre of the lens or the mirror The radii of curvature are positive (negative) when the respective lens or mirror surfaces are convex (concave). Dioptric power (strength of the lens): reciprocal value of focal length = D = S = 1/f [m -1 = dpt = D (dioptre)] Converging lenses: f and are positive Diverging lenses: f and are negative](https://image.slidesharecdn.com/visionbf-1h-fin-110914112002-phpapp02/85/Visionbf-1h-fin-9-320.jpg)
![Lens equation The rays parallel to the principal axis are refracted into the back focus (in converging lens), or so that they seem to be emitted from the front focus (in diverging lens). The direction of rays passing through the centre of the lens remains uninfluenced. Lens equation (equation of image, imaging equation): a – object distance [m] b – image distance [m] Sign convention: a is positive in front of the lens, negative behind the lens; b is negative in front of the lens (the image is virtual), positive behind the lens (the image is real)](https://image.slidesharecdn.com/visionbf-1h-fin-110914112002-phpapp02/85/Visionbf-1h-fin-10-320.jpg)
Visionbf 1h-fin
- 1. Biophysics of visual perception Lectures on Medical Biophysics Department of Biophysics, Medical Faculty, Masaryk University , Brno
- 2. Lecture outline Basic properties of light Anatomy of eye Optical properties of eye Retina – biological detector of the light Colour vision
- 3. Basic properties of light Visible electromagnetic radiation: λ = 380 – 790 nm shorter wavelength – U ltraviolet light ( UV ) longer wavelength – I nfrared light ( IR ) V isible light – ( VIS ) Medium in which the light propagates is called optical medium. In homogeneous media, light propagates in straight lines perpendicular to wave fronts, this lines are called light rays. Speed ( velocity ) of light ( in vacuum ) c = 299 792 458 ms -1 ap p rox. = 300 000 000 ms -1
- 4. Light (VIS) sources Natural Man – made ( artificial, forced, synthetic ) Natural: The sun It’s no accident that the main function of the sun at the centre of our solar system is to provide light. Light is what drives life. It’s hard to imagine our world and life without it. Man – made: light bulbs, fluorescent tubes, laser…
- 5. Polychromatic and Monochromatic Light Polychromatic or white light consists of light of a variety of wavelengths. Monochromatic light consists of light of a single wavelength A ccording to phase character light can be Coherent Coherent light are light waves "in phase" with one another. Light produced by lasers is coherent light. Incoherent Incoherent light are light waves that are not "in phase" with one another. Light from light bulbs or the sun is incoherent light.
- 6. Reflection and refraction of light Reflection - Law of reflection: The angle of reflection ’ equals to the angle of incidence . The ray reflected travels in the plane of incidence. Refraction: When light passes from one medium into another, the beam changes direction at the boundary between the two media. This property of optical media is characterised by index of refraction n = c/v [ dimensionless ] n – index of refraction of respective medium c – speed of light in vacuum v – speed of light in the respective medium index of refraction of vacuum is 1
- 7. Snell‘s law ( Law of refraction ) α – angle of incidence ( in medium 1) β – angle of refraction ( in medium 2 ) ( A ngles are measured away from the normal ! ) n 1 , n 2 – indices of refraction v 1 , v 2 , – s peed of light in respective media n is large: large optical density n is small: small optical density n 1 > n 2 – light refraction away the normal occurs n 1 < n 2 – light refraction toward the normal occurs Reflection and refraction of light
- 8. Lens–make r’ s equation f - focal distance (length) [m] n 2 - index of refraction of the lens n 1 - index of refraction of the medium r 1 , r 2 - radii of curvature of the lens
- 9. Common principles of optical imaging Real image (can be projected): convergence of rays Virtual image (cannot be projected): divergence of ray Principal axis – optical axis of centred system of optical boundaries Principal focus is a point where rays parallel to the principal axis intersect after refraction by the lens or reflection by the curved mirror - front ( object ) focus and back (image) focus Focal distance (length) f [m] is the distance of focus from the centre of the lens or the mirror The radii of curvature are positive (negative) when the respective lens or mirror surfaces are convex (concave). Dioptric power (strength of the lens): reciprocal value of focal length = D = S = 1/f [m -1 = dpt = D (dioptre)] Converging lenses: f and are positive Diverging lenses: f and are negative
- 10. Lens equation The rays parallel to the principal axis are refracted into the back focus (in converging lens), or so that they seem to be emitted from the front focus (in diverging lens). The direction of rays passing through the centre of the lens remains uninfluenced. Lens equation (equation of image, imaging equation): a – object distance [m] b – image distance [m] Sign convention: a is positive in front of the lens, negative behind the lens; b is negative in front of the lens (the image is virtual), positive behind the lens (the image is real)
- 11. The human eye can detect light from about 380 nm (violet) to about 790 nm (red). Our visual system perceives this range of light wavelength as a smoothly varying rainbow of colours. We call this range visible spectrum . The following illustration shows approximately how it is experienced . Visible spectrum
- 12. Anatomy of eye
- 13. How Does The Human Eye Work? The individual components of the eye work in a manner similar to a camera. Each part plays a vital role in providing clear vision. The Camera The Human Eye
- 14. Visual analyser consists of three parts: Eye – the best investigated part from the biophysical point of view Optical lines – channel which consists of nervous cells, through this channel the information registered and processed by the eye are given to the cerebrum Visual centre – the area of the cerebral cortex where is outwards picture perceived
- 15. Biophysics of visual perception Anatomy of the eyeball
- 16. Anatomy of the eyeball The tough, outermost layer of the eye is called the sclera . It maintains the shape of the eye. The front about sixth of this layer is clear and is called the cornea . All light must first pass through the cornea when it enters the eye. Attached to the sclera are the six muscles that move the eye, called the extraocular muscles . The chorioid (or uveal tract) is the second layer of the eye. It contains the blood vessels that supply blood to structures of the eye. The front part of the chorioid contains two structures: The ciliary body - t he ciliary body is a muscular area that is attached to the lens. It contracts and relaxes to control the curvature of the lens for focusing.
- 17. Anatomy of the eyeball The iris - t he iris is the colo u red part of the eye. The colour of the iris is determined by the colour of the connective tissue and pigment cells. Less pigment makes the eyes blue; more pigment makes the eyes brown. The iris is an adjustable diaphragm around an opening called the pupil . Inside the eyeball there are two fluid-filled sections separated by the lens. The larger, back section contains a clear, gel-like material called vitreous humour The smaller, front section contains a clear, watery material called aqueous humour The aqueous humour is divided into two sections called the anterior chamber (in front of the iris) and the posterior chamber (behind the iris). The aqueous humour is produced in the ciliary body
- 18. Anatomy of the eyeball The iris has two muscles: The m. dilator pupillae makes the iris smaller and therefore the pupil larger, allowing more light into the eye; the m. sphincter pupillae makes the iris larger and the pupil smaller, allowing less light into the eye. Pupil size can change from 2 millimetres to 8 millimetres. This means that by changing the size of the pupil, the eye can change the amount of light that enters it by 30 times.
- 19. Anatomy of the eyeball The transparent crystalline lens of the eye is located immediately behind the iris . It is a clear, bi-convex structure about 10 mm in diameter. The lens changes shape because it is attached to muscles in the ciliary body. When this ciliary muscle is relaxed , its diameter increases and the lens is flattened . contracted , its diameter is reduced, and the lens becomes more spherical . These changes enable the eye to adjust its focus between far objects and near objects. The crystalline lens is composed of 4 layers, from the surface to the center: capsule, subcapsular epithelium, cortex, nucleus
- 20. Intraocular pressure (production versus drainage of aqueous humour - dynamic balance) 2,66 kPa (20 mmHg) ± 0,3 kPa Changes greater than ± 0,3kPa are pat h ological
- 21. Optical properties of eye
- 22. Gullstrand model The eye is approximated as an centred optical system with ability of automatic focussing , however, this model does not consider certain differences in curvature of the front and back surface of cornea as well as the diferences of refraction indices of the core and periphery of the crystalline lens.
- 23. Gullstrand´s model of the eye – basic parameters Refraction Index: cornea................................ 1.376 aqueous humour............... 1.336 lens... ..................................1.413 vitreous humour.…………… 1.336 Dioptric power: cornea ................................ 42.7 D lens – inside eye................ 21.7 D eye (whole)........................ 60.5 D Radius of curvature: cornea ...................................... 7.8 mm lens – outer wall........ ............ 10.0 mm lens – inner wall..................... -6.0 mm Focus location: (measured from top of the cornea): front (object) focus.................... -14.99 mm back (image) focus ................... 23.90 mm retinae location.......................... 23.90 mm Allvar Gullstrand 18 5 2 – 1930 Nobel Award – 1911 Swedish ophthalmologist
- 24. Accommodation Accommodation is eye lens ability to change its dioptric power in dependence on distance of the observed object. Accommodation – allowed by increasing curvature of outer lens wall (J.E.Purkyně) Far point - punctum remotum (R) Farthest point of distinct vision without accommodation. Near point - punctum proximum (P) Nearest point of distinct vision with maximum accommodation.
- 25. Presbyopia (“after 40” vision) Old–age sight After age 40, and most noticeably after age 45, the human eye is affected by presbyopia , which results in greater difficulty maintaining a clear focus at a near distance with an eye which sees clearly at a far away distance . This is due to a lessening of flexibility of the crystalline lens, as well as to a weakening of the ciliary muscles which control lens focusing, both attributable to the aging process.
- 26. Decrement of accommodation ability in dependence on age
- 27. Retina – biological detector of the light R etina - the light-sensing p art of the eye. It contains rod cells , responsible for vision in low light, and cone cells , responsible for colour vision and detail. When light contacts these two types of cells, a series of complex chemical reactions occurs. The light- activated rhodopsin creates electrical impulses in the optic nerve. Generally, the outer segment of rods are long and thin, whereas the outer segment of cones are more cone-shaped. In the back of the eye, in the centre of the retina, is the macula lutea (yellow spot ) . In the centre of the macula is an area called the fovea centralis. This area contains only cones and is responsible for seeing fine detail clearly.
- 28. Blind spot Density of cones decreases from the yellow spot to the periphery of retina. The rod s have maximum density in a circle around the yellow spot (20 o from this spot). The nerve fibres transmitting the stimulation of photoreceptors converge to a place positioned nasally from the yellow spot. This place with no photoreceptors is called blind spot. rods cones temporal yellow spot nasal Blind spot Relative density of photoreceptors
- 29. Biophysics of visual perception Rods and cones The outer segment of a rod or a cone contains the photosensitive chemicals. In rods, this chemical is called rhodopsin. In cones, these chemicals are called colour pigments . The retina contains 100 million rods and 7 million cones.
- 30. Rhodopsin When light comes in contact with the photosensitive chemical rhodopsin (also called visual purple ) a photochemical reaction occurs . Rhodopsin is a complex of a protein called scot ( opsin ) and 11-cis-retinal - the latter is derived from vitamin A ( lack of vitamin A causes vision problems). Rhodopsin decomposes when it is exposed to light because light causes a physical change in the 11-cis-retinal, 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 . Here is a diagram of the chemical reaction we just discussed:
- 31. Biochemistry of rhodopsin: Rhodopsin
- 33. Optical illusions indicating the role of visual cortex in processing of visual information
- 34. Optical illusions indicating the role of visual cortex in processing of visual information
- 35. Electrical phenomena in retina The electrical activity of retina is closely connected with photochemical reactions taking place in photoreceptors after illumination. Early receptor potential Late receptor potential Electroretinography (ERG), recorded by means of two differential electrodes, measured voltage ranges from 100 to 400 V
- 36. Colour vision
- 37. Colour Vision The colour-responsive chemicals in the cones are called cone pigments and are very similar to the chemicals in the rods. The retinal portion of the chemical is the same, however the scotopsin is replaced with photopsins. Therefore, the colour-responsive pigments are made of retinal and photopsins. There are three kinds of colour-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 colour. The human eye can sense almost any gradation of colour when red, green and blue are mixed (originally Young-Helmholtz trichromatic theory).
- 38. Colour Vision x– red 650 nm, y– green 530 nm z – blue 460 nm x + y + z = 1
- 39. Colour Vision – spectral sensitivity “ Green-sensitive” or “M” cones “ Blue-sensitive” or “S” cones “ Red-sensitive” or “L” cones
- 41. Colour Vision PHOTOPIC VISION normal vision in daylight; vision with sufficient illumination that the cones are active and hue is perceived Maximum at 555 nm, brightness over 100 cd .m -2 SCOTOPIC VISION the ability to see in reduced illumination (as in moonlight) Maximum at 507 nm Purkinje effect (The tendency of the peak sensitivity of the human eye to shift toward the blue end of the spectrum at low illumination levels.) J. E. Purkyně
- 42. Colour Vision Trichromates - have normal colour vision Monochromates - have only one cone colour sensing system Dichromates: protanopia (difficult distinguishing between blue/green and red/green) – „ red blindness “ deuteranopia (difficult distinguishing between red/purple and green/purple) – „ green blindness “ tritanopia (difficult distinguishing between yellow/green and blue/green) – „ blue blindness “
- 43. Investigation of colour vision pseudoisochromatic tables
- 44. Limits of vision visual acuity: given by angle of 1min. of arc (tested by Snellen's charts ) sensitivity (intensity ) limit: 2 – 3 photons in several ms frequency: 5 - 60 Hz depending on brightness wavelength limit about: 380 – 790 nm limit of stereoscopic vision: stereoscopic parallax difference smaller than 20 second of arc
- 45. Authors: Vojtěch Mornstein, Lenka For ýtková Content collaboration and language revision: Ivo Hrazdira, Carmel J. Caruana Presentation design: - Last revision: September 200 8
Editor's Notes
- #14: How Does The Human Eye Work? The individual components of the eye work in a manner similar to a camera. Each part plays a vital role in providing clear vision. So think of the eye as a camera with the cornea, behaving much like a lens cover. As the eye's main focusing element, the cornea takes widely diverging rays of light and bends them through the pupil, the dark, round opening in the center of the colored iris. The iris and pupil act like the aperture of a camera. Next in line is the lens which acts like the lens in a camera, helping to focus light to the back of the eye. Note that the lens is the part which becomes cloudy and is removed during cataract surgery to be replaced by an artificial implant nowadays. The Camera The Human Eye The very back of the eye is lined with a layer called the retina which acts very much like the film of the camera. The retina is a membrane containing photoreceptor nerve cells that lines the inside back wall of the eye. The photoreceptor nerve cells of the retina change the light rays into electrical impulses and send them through the optic nerve to the brain where an image is perceived. The center 10% of the retina is called the macula. This is responsible for your sharp vision, your reading vision. The peripheral retina is responsible for the peripheral vision. As with the camera, if the &quot;film&quot; is bad in the eye (i.e. the retina), no matter how good the rest of the eye is, you will not get a good picture. The human eye is remarkable. It accommodates to changing lighting conditions and focuses light rays originating from various distances from the eye. When all of the components of the eye function properly, light is converted to impulses and conveyed to the brain where an image is perceived. Glossary of Eye Terms: Anterior Chamber The cavity in the front part of the eye between the lens and cornea is called the Anterior Chamber. It is filled with Aqueous, a water-like fluid. This fluid is produced by the ciliary body and drains back into the blood circulation through channels in the chamber angle. It is turned over every100 minutes. Chamber Angle Located at the junction of the cornea, iris, and sclera, the anterior chamber angle extends 360 degrees at the perimeter of the iris. Channels here allow aqueous fluid to drain back into the blood circulation from the eye. May be obstructed in glaucoma. Ciliary Body A structure located behind the iris (rarely visible) which produces aqueous fluid that fills the front part of the eye and thus maintains the eye pressure. It also allows focusing of the lens. Conjunctiva A thin lining over the sclera, or white part of the eye. This also lines the inside of the eyelids. Cell in the conjunctiva produce mucous, which helps to lubricate the eye. Cornea The transparent, outer &quot;window&quot; and primary focusing element of the eye. The outer layer of the cornea is known as epithelium. Its main job is to protect the eye. The epithelium is made up of transparent cells that have the ability to regenerate quickly. The inner layer of the cornea is also made up of transparent tissue, which allows light to pass. Hyaloid Canal A narrow channel that runs from the optic disc to the back surface of the lens. It serves an embryologic function prior to birth but none afterwards. Iris Inside the anterior chamber is the iris. This is the part of the eye which is responsible for one's eye color. It acts like the diaphragm of a camera, dilating and constricting the pupil to allow more or less light into the eye. Pupil The dark opening in the center of the colored iris that controls how much light enters the eye. The colored iris functions like the iris of a camera, opening and closing, to control the amount of light entering through the pupil. Lens The part of the eye immediately behind the iris that performs delicate focusing of light rays upon the retina. In persons under 40, the lens is soft and pliable, allowing for fine focusing from a wide variety of distances. For individuals over 40, the lens begins to become less pliable, making focusing upon objects near to the eye more difficult. This is known as presbyopia. Macula The part of the retina which is most sensitive, and is responsible for the central (or reading) vision. It is located near the optic nerve directly at the back of the eye (on the inside). This area is also responsible for color vision. Optic Disc The position in the back of the eye where the nerve (along with an artery and vein) enters the eye corresponds to the &quot;blind spot&quot; since there are no rods or cones in these location. Normally, a person does not notice this blind spot since rapid movements of the eye and processing in the brain compensate for this absent information. This is the area that the ophthalmologist studies when evaluating a patient for glaucoma, a condition where the optic nerve becomes damaged often due to high pressure within the eye. As it looks like a cup when viewed with an ophthalmoscope, it is sometimes referred to as the Optic Cup. Optic Nerve The optic nerve is the structure which takes the information from the retina as electrical signals and delivers it to the brain where this information is interpreted as a visual image. The optic nerve consists of a bundle of about one million nerve fibers. Retina The membrane lining the back of the eye that contains photoreceptor cells. These photoreceptor nerve cells react to the presence and intensity of light by sending an impulse to the brain via the optic nerve. In the brain, the multitude of nerve impulses received from the photoreceptor cells in the retina are assimilated into an image. Sclera The white, tough wall of the eye. Few diseases affect this layer. It is covered by the episclera (a fibrous layer between the conjunctiva and sclera ) and conjunctiva, and eye muscles are connected to this. Vitreous Next in our voyage through the eye is the vitreous. This is a jelly-like substance that fills the body of the eye. It is normally clear. In early life, it is firmly attached to the retina behind it. With age, the vitreous becomes more water-like and may detach from the retina. Often, little clumps or strands of the jelly form and cast shadows which are perceived as &quot;floaters&quot;. While frequently benign, sometimes floaters can be a sign of a more serious condition such as a retinal tear or detachment and should be investigated with a thorough ophthalmologic examination. How Does The Human Eye Work? The individual components of the eye work in a manner similar to a camera. Each part plays a vital role in providing clear vision. So think of the eye as a camera with the cornea, behaving much like a lens cover. As the eye's main focusing element, the cornea takes widely diverging rays of light and bends them through the pupil, the dark, round opening in the center of the colored iris. The iris and pupil act like the aperture of a camera. Next in line is the lens which acts like the lens in a camera, helping to focus light to the back of the eye. Note that the lens is the part which becomes cloudy and is removed during cataract surgery to be replaced by an artificial implant nowadays. The Camera The Human Eye The very back of the eye is lined with a layer called the retina which acts very much like the film of the camera. The retina is a membrane containing photoreceptor nerve cells that lines the inside back wall of the eye. The photoreceptor nerve cells of the retina change the light rays into electrical impulses and send them through the optic nerve to the brain where an image is perceived. The center 10% of the retina is called the macula. This is responsible for your sharp vision, your reading vision. The peripheral retina is responsible for the peripheral vision. As with the camera, if the &quot;film&quot; is bad in the eye (i.e. the retina), no matter how good the rest of the eye is, you will not get a good picture. The human eye is remarkable. It accommodates to changing lighting conditions and focuses light rays originating from various distances from the eye. When all of the components of the eye function properly, light is converted to impulses and conveyed to the brain where an image is perceived. Glossary of Eye Terms: Anterior Chamber The cavity in the front part of the eye between the lens and cornea is called the Anterior Chamber. It is filled with Aqueous, a water-like fluid. This fluid is produced by the ciliary body and drains back into the blood circulation through channels in the chamber angle. It is turned over every100 minutes. Chamber Angle Located at the junction of the cornea, iris, and sclera, the anterior chamber angle extends 360 degrees at the perimeter of the iris. Channels here allow aqueous fluid to drain back into the blood circulation from the eye. May be obstructed in glaucoma. Ciliary Body A structure located behind the iris (rarely visible) which produces aqueous fluid that fills the front part of the eye and thus maintains the eye pressure. It also allows focusing of the lens. Conjunctiva A thin lining over the sclera, or white part of the eye. This also lines the inside of the eyelids. Cell in the conjunctiva produce mucous, which helps to lubricate the eye. Cornea The transparent, outer &quot;window&quot; and primary focusing element of the eye. The outer layer of the cornea is known as epithelium. Its main job is to protect the eye. The epithelium is made up of transparent cells that have the ability to regenerate quickly. The inner layer of the cornea is also made up of transparent tissue, which allows light to pass. Hyaloid Canal A narrow channel that runs from the optic disc to the back surface of the lens. It serves an embryologic function prior to birth but none afterwards. Iris Inside the anterior chamber is the iris. This is the part of the eye which is responsible for one's eye color. It acts like the diaphragm of a camera, dilating and constricting the pupil to allow more or less light into the eye. Pupil The dark opening in the center of the colored iris that controls how much light enters the eye. The colored iris functions like the iris of a camera, opening and closing, to control the amount of light entering through the pupil. Lens The part of the eye immediately behind the iris that performs delicate focusing of light rays upon the retina. In persons under 40, the lens is soft and pliable, allowing for fine focusing from a wide variety of distances. For individuals over 40, the lens begins to become less pliable, making focusing upon objects near to the eye more difficult. This is known as presbyopia. Macula The part of the retina which is most sensitive, and is responsible for the central (or reading) vision. It is located near the optic nerve directly at the back of the eye (on the inside). This area is also responsible for color vision. Optic Disc The position in the back of the eye where the nerve (along with an artery and vein) enters the eye corresponds to the &quot;blind spot&quot; since there are no rods or cones in these location. Normally, a person does not notice this blind spot since rapid movements of the eye and processing in the brain compensate for this absent information. This is the area that the ophthalmologist studies when evaluating a patient for glaucoma, a condition where the optic nerve becomes damaged often due to high pressure within the eye. As it looks like a cup when viewed with an ophthalmoscope, it is sometimes referred to as the Optic Cup. Optic Nerve The optic nerve is the structure which takes the information from the retina as electrical signals and delivers it to the brain where this information is interpreted as a visual image. The optic nerve consists of a bundle of about one million nerve fibers. Retina The membrane lining the back of the eye that contains photoreceptor cells. These photoreceptor nerve cells react to the presence and intensity of light by sending an impulse to the brain via the optic nerve. In the brain, the multitude of nerve impulses received from the photoreceptor cells in the retina are assimilated into an image. Sclera The white, tough wall of the eye. Few diseases affect this layer. It is covered by the episclera (a fibrous layer between the conjunctiva and sclera ) and conjunctiva, and eye muscles are connected to this. Vitreous Next in our voyage through the eye is the vitreous. This is a jelly-like substance that fills the body of the eye. It is normally clear. In early life, it is firmly attached to the retina behind it. With age, the vitreous becomes more water-like and may detach from the retina. Often, little clumps or strands of the jelly form and cast shadows which are perceived as &quot;floaters&quot;. While frequently benign, sometimes floaters can be a sign of a more serious condition such as a retinal tear or detachment and should be investigated with a thorough ophthalmologic examination. How Does The Human Eye Work? The individual components of the eye work in a manner similar to a camera. Each part plays a vital role in providing clear vision. So think of the eye as a camera with the cornea, behaving much like a lens cover. As the eye's main focusing element, the cornea takes widely diverging rays of light and bends them through the pupil, the dark, round opening in the center of the colored iris. The iris and pupil act like the aperture of a camera. Next in line is the lens which acts like the lens in a camera, helping to focus light to the back of the eye. Note that the lens is the part which becomes cloudy and is removed during cataract surgery to be replaced by an artificial implant nowadays. The Camera The Human Eye The very back of the eye is lined with a layer called the retina which acts very much like the film of the camera. The retina is a membrane containing photoreceptor nerve cells that lines the inside back wall of the eye. The photoreceptor nerve cells of the retina change the light rays into electrical impulses and send them through the optic nerve to the brain where an image is perceived. The center 10% of the retina is called the macula. This is responsible for your sharp vision, your reading vision. The peripheral retina is responsible for the peripheral vision. As with the camera, if the &quot;film&quot; is bad in the eye (i.e. the retina), no matter how good the rest of the eye is, you will not get a good picture. The human eye is remarkable. It accommodates to changing lighting conditions and focuses light rays originating from various distances from the eye. When all of the components of the eye function properly, light is converted to impulses and conveyed to the brain where an image is perceived. Glossary of Eye Terms: Anterior Chamber The cavity in the front part of the eye between the lens and cornea is called the Anterior Chamber. It is filled with Aqueous, a water-like fluid. This fluid is produced by the ciliary body and drains back into the blood circulation through channels in the chamber angle. It is turned over every100 minutes. Chamber Angle Located at the junction of the cornea, iris, and sclera, the anterior chamber angle extends 360 degrees at the perimeter of the iris. Channels here allow aqueous fluid to drain back into the blood circulation from the eye. May be obstructed in glaucoma. Ciliary Body A structure located behind the iris (rarely visible) which produces aqueous fluid that fills the front part of the eye and thus maintains the eye pressure. It also allows focusing of the lens. Conjunctiva A thin lining over the sclera, or white part of the eye. This also lines the inside of the eyelids. Cell in the conjunctiva produce mucous, which helps to lubricate the eye. Cornea The transparent, outer &quot;window&quot; and primary focusing element of the eye. The outer layer of the cornea is known as epithelium. Its main job is to protect the eye. The epithelium is made up of transparent cells that have the ability to regenerate quickly. The inner layer of the cornea is also made up of transparent tissue, which allows light to pass. Hyaloid Canal A narrow channel that runs from the optic disc to the back surface of the lens. It serves an embryologic function prior to birth but none afterwards. Iris Inside the anterior chamber is the iris. This is the part of the eye which is responsible for one's eye color. It acts like the diaphragm of a camera, dilating and constricting the pupil to allow more or less light into the eye. Pupil The dark opening in the center of the colored iris that controls how much light enters the eye. The colored iris functions like the iris of a camera, opening and closing, to control the amount of light entering through the pupil. Lens The part of the eye immediately behind the iris that performs delicate focusing of light rays upon the retina. In persons under 40, the lens is soft and pliable, allowing for fine focusing from a wide variety of distances. For individuals over 40, the lens begins to become less pliable, making focusing upon objects near to the eye more difficult. This is known as presbyopia. Macula The part of the retina which is most sensitive, and is responsible for the central (or reading) vision. It is located near the optic nerve directly at the back of the eye (on the inside). This area is also responsible for color vision. Optic Disc The position in the back of the eye where the nerve (along with an artery and vein) enters the eye corresponds to the &quot;blind spot&quot; since there are no rods or cones in these location. Normally, a person does not notice this blind spot since rapid movements of the eye and processing in the brain compensate for this absent information. This is the area that the ophthalmologist studies when evaluating a patient for glaucoma, a condition where the optic nerve becomes damaged often due to high pressure within the eye. As it looks like a cup when viewed with an ophthalmoscope, it is sometimes referred to as the Optic Cup. Optic Nerve The optic nerve is the structure which takes the information from the retina as electrical signals and delivers it to the brain where this information is interpreted as a visual image. The optic nerve consists of a bundle of about one million nerve fibers. Retina The membrane lining the back of the eye that contains photoreceptor cells. These photoreceptor nerve cells react to the presence and intensity of light by sending an impulse to the brain via the optic nerve. In the brain, the multitude of nerve impulses received from the photoreceptor cells in the retina are assimilated into an image. Sclera The white, tough wall of the eye. Few diseases affect this layer. It is covered by the episclera (a fibrous layer between the conjunctiva and sclera ) and conjunctiva, and eye muscles are connected to this. Vitreous Next in our voyage through the eye is the vitreous. This is a jelly-like substance that fills the body of the eye. It is normally clear. In early life, it is firmly attached to the retina behind it. With age, the vitreous becomes more water-like and may detach from the retina. Often, little clumps or strands of the jelly form and cast shadows which are perceived as &quot;floaters&quot;. While frequently benign, sometimes floaters can be a sign of a more serious condition such as a retinal tear or detachment and should be investigated with a thorough ophthalmologic examination.