 |
|
|
| Shared Idea |
 |
| Vision, Optics and Color Perception
|
| The human eye plays the central role in our perception of vision and it has a variety of unique traits. For example, the surface of the eye is the cornea, which allows light to enter this complex sensory organ. No blood capillaries pass through these cells, as they would interfere with our sense of vision. Nonetheless, nutrients need to enter these cells and wastes need to be removed. These processes are carried out when we blink, an involuntary mechanism that is necessary for the cornea to be able to function properly.
|
| The image passing through the eye lens is inverted. This means that an upside-down picture falls on the rods and cones lining the back of the eye. Rods perceive black and white while cones perceive color. The message reaching the brain inverts this so that it is pictured as being right-side-up. When experiments were done with lenses that inverted this image, nausea and discomfort followed. However, it was found that after three days the brain began adjusting to perceive the image correctly, taking one to two weeks to return vision to the normal perception.
|
| Unlike cephalopods, such as the octopus, the nerves from the rods and cones in vertebrates are inside the eye. They meet together and pass out of each eye as an optic nerve, bringing the visual information to the brain. This means that there is a blind spot in each eye, a region where no information is sent to the occipital lobe of the brain. The brain "paints in" this region based on the information received from the neighboring regions. Stephen Jay Gould cited this fact as primary evidence of natural selection, favoring evolution over divine design.
|
| To test this concept, take a sheet of white paper and draw two small black circles several inches apart from one another. Close or cover one eye and align the matching dot in front of the open eye. Slowly move the sheet away from your face, focusing on that single dot. At a certain distance, the second dot will disappear. When you move a bit further, it will reappear. It "vanished" when it was situated in the blind spot, as the brain was only receiving the image of the surrounding white paper. This can be repeated with the other eye, making you aware of your second blind spot.
|
| The three primary colors of paint are red, blue and yellow. However, the three primary colors of light are red, blue and green. These are the colors perceived by the cones in our eyes. If you look closely at the screen of an early color television, you will see the same three colors. Changing the proportion of each of the cones creates the different colors we can distinguish. For the average human, that is between one and ten million different colors!
|
| When you see a blue surface, it is actually orange. Your eye sees the colors that are not absorbed, which is therefore the opposite of its actual color. The real world matches the colors seen on a color film negative. However, when you look at a traffic light, you are seeing the actual color of the light, as it is not reflected. A surface that appears black is absorbing all of the light, meaning it is actually white, which explains why it warms up when exposed to sunlight. The color of a person’s skin is actually the opposite of what you see!
|
| If you are watching a television or movie screen, you are actually seeing a sequential set of pictures. Your brain then blends these pictures to see a life-like image. If, on the other hand, you stare at a single solid color, the cones that are receiving the light begin to tire out, which can influence what you are seeing. This is easy to test. Take two white index cards. Draw a circle of a color other than yellow about the size of a quarter and color it in. (This means you are not selecting yellow, black, grey or brown.) Place this card over the white card and stare at it for 20 to 30 seconds and then remove the upper card. On the white card, you will see a ring of the opposite color as the cones are not equally tired.
|
| Everyone has seen a rainbow. However, it is not in the sky. If you try to approach it, it seems to move away. This happens because two things are needed to create a rainbow. First, you need something that diffracts the light, such as the water droplets in the sky after it rains. You then need a lens to focus the different wavelengths of the light, thereby creating the rainbow. This means that the rainbow is actually inside your eye! Your brain visualizes the rainbow with whatever background you are viewing. (Sorry, but you can't find gold at the end of the rainbow.) You can confirm this by viewing a ceiling light through a diffraction grating. You will see the rainbow spectrum, but only when viewing through the grating. Those who do not have a diffraction grating will only see white.
|
| If the rainbow isn't in the sky, why can you take a picture of it? The answer is actually quite simple. The camera has a lens. This means that the rainbow is also inside of the camera!
|
| A couple of interesting activities can close this session. Take a reverse image of an American flag. Place it atop a white sheet and stare at it for the short period used earlier. When you remove it, you will see the actual flag on the white sheet. Also, you can take a variety of optical illusions and experiment to see how they can influence your eye's perceptions. Many such illusions are quite remarkable!
|
|
|
