Why Can't Humans See Ultraviolet Light?
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Ultraviolet Light
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Light rays, like sound, are emitted at a frequency. Ultraviolet lights are emitted at a frequency that is higher than the range the human eye can pick up, which means the waves are very small and tight. If you were to take your television remote control and look at the black section on the front when you hit the power button, you would not see the laser shoot from your controller to your television. The beam is of ultraviolet light. The lens of your digital camera can see the light rays because the camera doesn't have muscles and reflexes controlling what it can and can't see.
Why Human Eyes Cannot See It
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The human eye cannot process ultraviolet light waves because they are short. Human eyes can only see between 380 nm (violet light) and 750 nm (red light). The human eye is composed of two types of photoreceptors called rod cells and cone cells. Each of these cells are receptive to different wavelengths of light--short, medium and long. However, none of the three are receptive to the short wavelengths of ultraviolet light.
Birds and other animals can see ultraviolet life because their vision differs from humans. Ultraviolet light receptors have been found in analysis of diurnal birds, having been found in over 40 species using a combination of microspectrophotometry, electrophysiology, behavioral methods and gene sequencing.
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Anatomy of the Human Eye
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Human eyes were engineered to focus light onto the retina. Once light hits the retina, it causes chemical changes in the photosensitive cells of the retina, which triggers nerve impulses that travel to the brain via nerve cells.
Light enters the pupil and is focused on the retina by the lens. Light-sensitive nerve cells called rods, which control brightness, and cones, which control color, react to the light. They interact with each other and send messages to the brain. The rods and cones enable vision. The pupil dilates and expands to meet the needs of the cells. This is why your eyes adjust to light and darkness.
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