How the eye works

How the eye works
You probably take your eyes for granted. But how do they work? Your eye is very similar to a magnifying glass: A converging lens at the entrance to the eye focuses light into an image on the back of the eye (called the retina). Read the text aloud Show Eye combines two lenses
How does your eye manage to see both in bright sunlight and dark nighttime? The pupil is an entrance hole that can vary its size to change the amount of light entering the eye. The iris dilates—opens the pupil wider—in dim conditions to let in more light. It contracts to block out some light in bright conditions. Read the text aloud
One amazing feature of the eye is that the lens can vary its shape! The purpose of the lens is to form an image on the retina. If you want to look at a nearby book—or distant trees—the ciliary muscles in your eye will change the shape of its lens to change its focal length. The eye thus maintains the image location on the retina. Accommodation is the eye’s process of changing the focal length of its lens. Read the text aloud Show Changing the image location using the thin lens formula
At the retina, the eye has two kinds of photoreceptors called rods and cones that detect light. When light hits a photoreceptor cell, it initiates a chemical process that in turn creates an electrical signal that is sent to the brain. Most of the eye’s photoreceptor cells are rods, which detect the intensity of light but not its color. The rods tell the brain how bright the light is at every point across your retina. Read the text aloud
But your eyes don’t see in black and white—they see in color! There are three different kinds of photoreceptor cells called cones: red, green, and blue. The cones in your eye are similar to the RGB color emission process in a computer monitor! When red light strikes your retina, only the red cones are stimulated; with cyan light, the blue and green cones are stimulated. A typical human eye has approximately 125 million rod cells but only 6 million cones. Since there are relatively few cone cells, the eye cannot distinguish among colors at low light levels. Read the text aloud
Imagine that the eye is like a microscope. Which action of adjusting a microscope is most like the way an eye focuses?
  1. changing the height of the slide with the focus to bring it to the focal point
  2. adjusting the power of the lamp so the subject can be easily seen
  3. moving the sample slide so it is directly above the lamp
  4. switching the objective lens for the lens with the optimal focal point
Although it may not be intuitive, because it is unlike paint mixing, why is RGB additive mixing a helpful color model? Show

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