The optic disk is a blind spot because it does not contain any rod or cone cells (the light sensitive cells on the retina). Looking at the cross focuses it onto the macula. The image of the circle disappears from view when its image is focused onto the optic disk. This happens at a distance which depends on the separation between the cross and the circle, and the distance between the macula and the optic disk. We don’t notice the blind spot in our field of vision because the brain ‘fills in the blanks’.
How is an Eye like a Camera?
Light Control
Light Control
- A camera functions very much like the human eye. Both see light that is reflected from the object being viewed. It transmits itself to the eye as sight or the camera as a picture in film or digital form. Light rays diffuse and bounce randomly from the myriad of objects around us so in order to see or form a picture, the rays need to be focused into a coherent form. To begin, the light enters the eye through the cornea and, like a camera's aperture, the pupil enlarges or contracts to ensure that the right amount of light enters the eye.
- When light enters the eye, it focuses on cells on an area at the back of the eye that is sensitive to light, which is known as the retina. Just as a camera responds to light entering it to form an image on film or digitally, the retina forms an image by transmitting the light to the brain by means of the optic nerve.
- Light rays bend when they enter the camera so that an image is upside down, a situation that's corrected when the picture is formed digitally or in film processing. The same is true of the light rays that bend on the retina but in that case, the brain actually does the work of correcting the image for us.
- The human eye, like a camera, can view tones of gray and the various shades of color (unless the person happens to be color blind, of course.) Both the eye and the camera can see near and far, judge size, register depth perception and see movement. However, the human eye can only see in visible light. Specialized cameras, films and digital photography can go much further by capturing images that are far beyond the eye's capability. The camera can show heat images and X-ray views, stop motion or capture images too fast for the human eye, such as a bullet in mid-flight, and cameras can be designed to view everything from the smallest particle to the planets at extreme distances.
Fixate on the crosshairs. After 20 seconds or so, the fuzzy lilac dots fade to gray. The absence of a dot, which hops around the chain, becomes a rotating dot of green.
This visual trickery is called Troxler's fading, or Troxler's effect, and was discovered by Swiss polymath Ignaz Paul Vital Troxler in 1804. The effect results from the ability of our visual neurons to switch off their awareness of things that aren't changing, and heighten their perception of things that are. In the footage, the lilac dots stay still while the absence of the dots moves. Thus, after a brief figuring-out period, the visual system transitions to focusing on only the moving blank dots which it turns green because of a second illusion at play here and lets the immobile lilac dots fade.
Other human sensory systems behave similarly. If a bug lands on your arm, for example, you can feel it at first. But if it stands still for a few seconds, you lose the physical sensation of its presence. Only when it keeps walking, giving varying stimulation to your tactile neurons, do you keep feeling it.
As for the other optical illusion, the blank dot turns minty green because your retina has been oversaturated with the lilac colored dots. When the lilac is removed from the spots, you see complementary color (minty green) instead, which is composed of white light minus the lilac.
Nothing is moving here. Promise.
There's no solid explanation for illusory motion. Some visual scientists think it has to do with fixation jitter: involuntary eye movements that give the illusion that objects near what you're fixated on are moving. Others think that when you glance around the image, motion detectors in your visual cortex get "confused" by dynamical changes in neurons, and think you're seeing movements.
This visual trickery is called Troxler's fading, or Troxler's effect, and was discovered by Swiss polymath Ignaz Paul Vital Troxler in 1804. The effect results from the ability of our visual neurons to switch off their awareness of things that aren't changing, and heighten their perception of things that are. In the footage, the lilac dots stay still while the absence of the dots moves. Thus, after a brief figuring-out period, the visual system transitions to focusing on only the moving blank dots which it turns green because of a second illusion at play here and lets the immobile lilac dots fade.
Other human sensory systems behave similarly. If a bug lands on your arm, for example, you can feel it at first. But if it stands still for a few seconds, you lose the physical sensation of its presence. Only when it keeps walking, giving varying stimulation to your tactile neurons, do you keep feeling it.
As for the other optical illusion, the blank dot turns minty green because your retina has been oversaturated with the lilac colored dots. When the lilac is removed from the spots, you see complementary color (minty green) instead, which is composed of white light minus the lilac.
Nothing is moving here. Promise.
There's no solid explanation for illusory motion. Some visual scientists think it has to do with fixation jitter: involuntary eye movements that give the illusion that objects near what you're fixated on are moving. Others think that when you glance around the image, motion detectors in your visual cortex get "confused" by dynamical changes in neurons, and think you're seeing movements.
The horizontal bar in image below looks gradated, moving from light to dark grey in the opposite direction as the background. You may have already guessed it: This is just a trick of the mind. If you cover everything but the bar itself, you'll see that it's actually monochrome.The brain interprets the two ends of the bar as being under different illuminations, and deduces what it thinks the bar's true shading would be (if it were lit evenly along its length). It deduces that the left end of the bar is a light gray object in dim lighting. The right end looks like a darker object that is well-lit.
In this geometrical-optical illusion two straight and parallel lines look as if they bow outwards. Our brains overestimate the angle made at the points of intersection between the radiating lines and the red ones. But why do we miscalculate? It has to do with the human tendency to visually predict the near future. Because there's a lag between the time that light hits the retina and the time when the brain perceives that light, the human visual system has evolved to compensate for the neural delay by generating images of what will occur one-tenth of a second into the future.
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Your Assignment: The world would look different to you if you had different eyes. A camera can help us see things in a different way, from a different point of view than what we are used to. At this stage, we are not ready to play around with the camera settings to make the camera see things differently than our eyes do, but we can still take some interesting photographs by using our camera from a view that we are not used to. Take at least 4 pictures while pretending that you are seeing through the eyes of different animals and people
1 Bird’s-eye view -- an elevated view of an object from above, with a perspective as though the observer were a bird
2 Worm’s-eye view -- It’s not a common view that people see, so images you create shot from this angle have a unique look and pique interest with viewers. Your job is to take on this low down task and shoot from the ground level.
3 Mystery view: Pretend you’re a different animal and take a picture from that point of view. Let's see if I can guess what animal it is
4 Your eye view or a photo taken with you at the head of the class as a teacher
1 Bird’s-eye view -- an elevated view of an object from above, with a perspective as though the observer were a bird
2 Worm’s-eye view -- It’s not a common view that people see, so images you create shot from this angle have a unique look and pique interest with viewers. Your job is to take on this low down task and shoot from the ground level.
3 Mystery view: Pretend you’re a different animal and take a picture from that point of view. Let's see if I can guess what animal it is
4 Your eye view or a photo taken with you at the head of the class as a teacher