Vision Eyesight The eye is an amazing combination of physics and biology.Only by understanding image production and nerves can we truly understand what we see.Use the outline below,or the buttons at the bottom to explore the eye. ·Parts of the eye Sensing light with nerves Lens ·Image formation ·Near sighted Iris ·Far sighted Optic Nerve Retina Limitations and oddities Cornea Parts of the Eye The eye is made up of 5 major parts.The image below shows the 1000 Scienee Joy wagon cross section of an eye with the five parts labeled.Light enters through the clear covering of the eye called the cornea.It then passes through the adjustable opening in the eye called the iris.(the colored part)The lens,which is a bit soft and flexible,is supposed to focus the image of the object onto the retina.The retina is like the movie screen of the eye.Any image formed on the retina should cause nerves to fire sending a signal along the optic nerve to be "seen"by the mind. Cornea The clear covering of the eye.If the cornea is mis-shaped,it may lead to vision trouble. Iris
Vision & Eyesight The eye is an amazing combination of physics and biology. Only by understanding image production and nerves can we truly understand what we see. Use the outline below, or the buttons at the bottom to explore the eye. • Parts of the eye • Sensing light with nerves • Image formation • Near sighted • Far sighted • Limitations and oddities Parts of the Eye The eye is made up of 5 major parts. The image below shows the cross section of an eye with the five parts labeled. Light enters through the clear covering of the eye called the cornea. It then passes through the adjustable opening in the eye called the iris. (the colored part) The lens, which is a bit soft and flexible, is supposed to focus the image of the object onto the retina. The retina is like the movie screen of the eye. Any image formed on the retina should cause nerves to fire sending a signal along the optic nerve to be "seen" by the mind. Cornea The clear covering of the eye. If the cornea is mis-shaped, it may lead to vision trouble. Iris
The colored opening in the eye.The iris'job is to open or close to adjust the amount of light that actually enters the eye. Lens The lens is a converging lens so that it can create a real image on the back wall of the eye.The lens is actually soft in the middle (like a bag of water)so that the muscles around it can adjust its shape and focus.This allows us to focus on something close to us and far away. Retina The projection screen at the back of the eye.If the image gets focused here, vision is good. Optic Nerve The nerve that carries the signal from the retina to the brain.It is also the cause of the blind spot you have in each eye. Vision Eyesight:Sensing Light with Nerves The retina is made up of two types of nerve cells capable of detecting or sensing light. Rods Rods are nerves that are very good at sensing low levels of light.They actually become more active in low light situations.The drawback is that they only sense black and white or shades of gray.They are located heavily around the outer edges of the retina.(see image for placement details) Cones Cones are nerves that are very good at detecting color but require more light to function.They become less active in low light situations.They are located more toward the center of the retina.(see image for placement details).Cones come in three sub types:Red,Blue,and Green.Blue cones only sense shades of blue light, Red only sense reddish light ..etc
The colored opening in the eye. The iris' job is to open or close to adjust the amount of light that actually enters the eye. Lens The lens is a converging lens so that it can create a real image on the back wall of the eye. The lens is actually soft in the middle (like a bag of water) so that the muscles around it can adjust its shape and focus. This allows us to focus on something close to us and far away. Retina The projection screen at the back of the eye. If the image gets focused here, vision is good. Optic Nerve The nerve that carries the signal from the retina to the brain. It is also the cause of the blind spot you have in each eye. Vision & Eyesight: Sensing Light with Nerves The retina is made up of two types of nerve cells capable of detecting or sensing light. Rods Rods are nerves that are very good at sensing low levels of light. They actually become more active in low light situations. The drawback is that they only sense black and white or shades of gray. They are located heavily around the outer edges of the retina. (see image for placement details) Cones Cones are nerves that are very good at detecting color but require more light to function. They become less active in low light situations. They are located more toward the center of the retina. (see image for placement details). Cones come in three sub types: Red, Blue, and Green. Blue cones only sense shades of blue light, Red only sense reddish light ...etc
Rod Cone Placement Since the cones are not very good at low light levels,and they occupy most of the center of the retina it makes it difficult to see a poorly lit object at night if you look directly it. Looking slightly off to one side of the object or another,the image falls on the outer edge of the retina which makes it more visible at night. A chemical reaction is actually responsible for making the shift as the amount of light changes between the rods or cones being active.Waiting for this chemical reaction to occur is what we are doing when we let our eyes "adjust"as we go from light to dark,or dark to light.Red light by itself will actually not interfere with this chemical reaction.That's why red lights are used in low light settings because they will not cause the chemical reaction which prevent the rods from working. Red lights will not ruin your night vision.This chemical reaction can happen independently in each eye.If you close your right eye for several minutes then turn off the lights,your right eye will not need to adjust to the dark (it already adjusted).Law enforcement people are often trained to keep one eye closed while in the dark so that if a bright flash were used to temporarily blind them,they could simply switch eyes and continue to see well in the dark. Each and every eye has at least one blind spot.Where the optic nerve attaches to the back of the eye there are no rods or cones.On the diagram above,this appears as a white spot.Any image formed on that small spot are not detected.The blind spot test below can be used to demonstrate the presence of your blind spot. X
Since the cones are not very good at low light levels, and they occupy most of the center of the retina it makes it difficult to see a poorly lit object at night if you look directly it. Looking slightly off to one side of the object or another, the image falls on the outer edge of the retina which makes it more visible at night. A chemical reaction is actually responsible for making the shift as the amount of light changes between the rods or cones being active. Waiting for this chemical reaction to occur is what we are doing when we let our eyes "adjust" as we go from light to dark, or dark to light. Red light by itself will actually not interfere with this chemical reaction. That's why red lights are used in low light settings because they will not cause the chemical reaction which prevent the rods from working. Red lights will not ruin your night vision. This chemical reaction can happen independently in each eye. If you close your right eye for several minutes then turn off the lights, your right eye will not need to adjust to the dark (it already adjusted). Law enforcement people are often trained to keep one eye closed while in the dark so that if a bright flash were used to temporarily blind them, they could simply switch eyes and continue to see well in the dark. Each and every eye has at least one blind spot. Where the optic nerve attaches to the back of the eye there are no rods or cones. On the diagram above, this appears as a white spot. Any image formed on that small spot are not detected. The blind spot test below can be used to demonstrate the presence of your blind spot
To find your blind spot in your right eye: Close your left eye and get your right eye a few centimeters away from the screen right in front of the circle.Stare directly at the circle and slowly move your head back away from the screen.As you move back,there is a point,less than a meter away,where the X will disappear.The exact distance where it disappears depends on the size of your screen and your eye. To find the blind spot in your left eye: Close your right eye and get your left eye a few centimeters away from the screen right in front of the X.Stare directly at the X and slowly move your head back away from the screen.As you move back,there is a point,less than a meter away,where the circle will disappear. Vision Eyesight:Image Formation 1099 Science Joy wagon As with any image being formed,it takes two or more rays from the same spot on an object to cross or appear to cross.In this case,looking at two rays from the candle flame,we find that the rays cross right at the location of the retina. This image would be in focus.It is important to notice that the real image of the candle is actually upside down on the retina.Your brain actually takes care of flipping it so it seems to be right side up.Some experiments that have been done involve people wearing some glasses that make everything seem upside down.After
To find your blind spot in your right eye: Close your left eye and get your right eye a few centimeters away from the screen right in front of the circle. Stare directly at the circle and slowly move your head back away from the screen. As you move back, there is a point, less than a meter away, where the X will disappear. The exact distance where it disappears depends on the size of your screen and your eye. To find the blind spot in your left eye: Close your right eye and get your left eye a few centimeters away from the screen right in front of the X. Stare directly at the X and slowly move your head back away from the screen. As you move back, there is a point, less than a meter away, where the circle will disappear. Vision & Eyesight: Image Formation As with any image being formed, it takes two or more rays from the same spot on an object to cross or appear to cross. In this case, looking at two rays from the candle flame, we find that the rays cross right at the location of the retina. This image would be in focus. It is important to notice that the real image of the candle is actually upside down on the retina. Your brain actually takes care of flipping it so it seems to be right side up. Some experiments that have been done involve people wearing some glasses that make everything seem upside down. After
a few days,their brain flips the image.If they take the glasses off,their brain, after a delay,will flip it right side up again. Vision Eyesight:Nearsightedness 回1g0 Sclence Joy Wlago If the lens in the eye is too strong,or the cornea is too thick,or the eye is too long,it will cause the real image to get formed in front of the retina.This will cause a blurred image.We would call it nearsightedness (myopia).Uncorrected, nearsightedness basically means you can't see things from a distance. 回1g0 Science-toy iagon Since the problem is that the rays are converging too soon,we want to undo some of the convergence.Divergence undoes convergence.As a result,nearsightedness is usually corrected by placing a diverging lens in the front eye. Vision Eyesight:Farsightedness
a few days, their brain flips the image. If they take the glasses off, their brain, after a delay, will flip it right side up again. Vision & Eyesight: Nearsightedness If the lens in the eye is too strong, or the cornea is too thick, or the eye is too long, it will cause the real image to get formed in front of the retina. This will cause a blurred image. We would call it nearsightedness (myopia). Uncorrected, nearsightedness basically means you can't see things from a distance. Since the problem is that the rays are converging too soon, we want to undo some of the convergence. Divergence undoes convergence. As a result, nearsightedness is usually corrected by placing a diverging lens in the front eye. Vision & Eyesight: Farsightedness
©1 99g Science Joy Wlagon If the lens in the eye is too weak,or the cornea is too thin,or the eye is too short,it will cause the real image to get formed in beyond the retina.This will cause a blurred image.We would call it farsightedness (hyperopia).Uncorrected, farsightedness basically means you can't see things close up. 1999 Scienoe Joy inagon Since the problem is that the rays aren't converging soon enough,we need to increase the convergence.Farsightedness is usually corrected by placing a converging lens in the front eye. Vision Eyesight:Limitations Oddities Other than being blind in at least one spot on your eye (due to where the optic nerve attaches)it turns out there are several reasons we can not trust our eyes. Persistence of Vision
If the lens in the eye is too weak, or the cornea is too thin, or the eye is too short, it will cause the real image to get formed in beyond the retina. This will cause a blurred image. We would call it farsightedness (hyperopia). Uncorrected, farsightedness basically means you can't see things close up. Since the problem is that the rays aren't converging soon enough, we need to increase the convergence. Farsightedness is usually corrected by placing a converging lens in the front eye. Vision & Eyesight: Limitations & Oddities Other than being blind in at least one spot on your eye (due to where the optic nerve attaches) it turns out there are several reasons we can not trust our eyes. Persistence of Vision
The human eye does not instantly forget what it saw.It takes at least 1/24th of a second to forget what it saw.Television and movies only show us a new picture every 24th of a second.Since our eyes didn't forget the previous image yet,the new image gets blended with the previous.This gives us the illusion of smooth, realistic motion.If an image is really bright,it may leave a trace on your eye for a much longer period of time.Seeing spots after someone takes a flash picture is a good example of this. 3D Effects We see things in 3 dimensions(3D)because we can see them from two points of view at the same time.Our left eye gives us one point of view,while our right eye provides the other.Our brain overlaps these two points of view into one image that contains depth.The images above look the same but are actually just a little different.If you use the same method to see these that you would use to see those 3D dot pattern images (uncrossing your eyes to focus on something behind the screen)you should see this image in 3D with the pink candle being closer to you. If you close one eye it removes 3D viewing.We can still infer 3D info based on a couple of other visual clues
The human eye does not instantly forget what it saw. It takes at least 1/24th of a second to forget what it saw. Television and movies only show us a new picture every 24th of a second. Since our eyes didn't forget the previous image yet, the new image gets blended with the previous. This gives us the illusion of smooth, realistic motion. If an image is really bright, it may leave a trace on your eye for a much longer period of time. Seeing spots after someone takes a flash picture is a good example of this. 3D Effects We see things in 3 dimensions (3D) because we can see them from two points of view at the same time. Our left eye gives us one point of view, while our right eye provides the other. Our brain overlaps these two points of view into one image that contains depth. The images above look the same but are actually just a little different. If you use the same method to see these that you would use to see those 3D dot pattern images (uncrossing your eyes to focus on something behind the screen) you should see this image in 3D with the pink candle being closer to you. If you close one eye it removes 3D viewing. We can still infer 3D info based on a couple of other visual clues
Overlap clues give us 3D info.For example,since the pink candle at the left covers part of the blue candle we can infer that the pink candle is closer to us. Relative size clues can also give us some 3D info. Larger objects are assumed to be closer than smaller objects.This only works if you have some pre-existing experience with the size of the objects you are looking at. Focus clues can also give you some information about distance.The muscles in your eye that focus the lens give you info on how far away something is,but this is limited to distances that are only a few meters away. Astigmatism An astigmatism is an eye problem that results from a the lens or cornea being misshaped in a non-symmetrical way.This results in light in the horizontal plane having a different focus than light in the vertical plane.It is corrected by a non-spherical lens that has a different thickness as you go across the lens than up and down the lens
Overlap clues give us 3D info. For example, since the pink candle at the left covers part of the blue candle we can infer that the pink candle is closer to us. Relative size clues can also give us some 3D info. Larger objects are assumed to be closer than smaller objects. This only works if you have some pre-existing experience with the size of the objects you are looking at. Focus clues can also give you some information about distance. The muscles in your eye that focus the lens give you info on how far away something is, but this is limited to distances that are only a few meters away. Astigmatism An astigmatism is an eye problem that results from a the lens or cornea being misshaped in a non-symmetrical way. This results in light in the horizontal plane having a different focus than light in the vertical plane. It is corrected by a non-spherical lens that has a different thickness as you go across the lens than up and down the lens