Humans perceive light that ranges between approximately nm and nm. However, some other animals can detect wavelengths outside of the human range. For example, bees see near-ultraviolet light in order to locate nectar guides on flowers. Some non-avian reptiles sense infrared light such as heat that prey gives off. Wave amplitude is perceived as luminous intensity or brightness.
The standard unit of intensity of light is the candela, which is approximately the luminous intensity of one common candle. Light waves travel , km per second in a vacuum and somewhat slower in various media such as air and water. Those waves arrive at the eye as long red , medium green , and short blue waves. This effect is produced by light that stimulates the color receptors in the human eye equally. The apparent color of an object is actually the color or colors the object reflects.
Thus a red object reflects the red wavelengths in mixed white light and absorbs all other wavelengths of light. Many structures in the human eye, such as the cornea and fovea, process light so it can be deciphered by rods and cones in the retina.
The retina, a thin layer of cells located on the inner surface of the back of the eye, consists of photoreceptive cells, which are responsible for the transduction of light into nervous impulses. However, light does not enter the retina unaltered; it must first pass through other layers that process it so that it can be interpreted by the retina.
Retina : a The human eye is shown in cross section. The human eye contains structures, such as the cornea, iris, lens, and fovea, that process light so it can be deciphered by the retina. Other structures like the aqueous humor and the vitreous humor help maintain the shape of the eye. The retina contains photoreceptive cells. In the retina, light is converted into neural signals sent to the brain. The cornea, the front transparent layer of the eye, along with the crystalline lens, refract bend light to focus the image on the retina.
After passing through the cornea, light passes through the aqueous humour, which connects the cornea to the lens. This clear gelatinous mass also provides the corneal epithelium with nutrients and helps maintain the convex shape of the cornea. The iris, which is visible as the colored part of the eye, is a circular muscular ring lying between the lens and the aqueous humour that regulates the amount of light entering the eye.
Light passes through the center of the iris, the pupil, which actively adjusts its size to maintain a constant level of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the size of the pupil. In conditions of low light, the iris relaxes and the pupil enlarges. The main function of the lens is to focus light on the retina and fovea centralis. The lens is a transparent, convex structure located behind the cornea. On the other side of the lens is the vitreous humour, which lets light through without refraction, maintains the shape of the eye, and suspends the delicate lens.
The lens focuses and re-focuses light as the eye rests on near and far objects in the visual field. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through to focus it sharply on the retina.
With age comes the loss of the flexibility of the lens; a form of farsightedness called presbyopia results.
Presbyopia occurs because the image focuses behind the retina. It is a deficit similar to a different type of farsightedness, hyperopia, caused by an eyeball that is too short. For both defects, images in the distance are clear, but images nearby are blurry.
Myopia nearsightedness occurs when an eyeball is elongated and the image focus falls in front of the retina. It separates all the types of light by wavelength because that directly relates to how energetic the wave is. More energetic waves have shorter wavelengths while less energetic waves have longer wavelengths. Not all light is in the visible spectrum , which is the light you can see.
There are many kinds of electromagnetic waves that you can't see. Just look at the spectrum below. What is a "detector"? When we talk about a "light detector", we are referring to any instrument that detects electromagnetic radiation. For example, your eye is a type of light detector.
It detects white visible light, which contains all the colors of the rainbow from red to violet. To many animal advocates, this would seem brutal, abusive, and unethical. What if you could do research that would help ensure babies and children born with certain conditions could develop normal vision instead of becoming blind?
Would you want that research done? Would you conduct that research, even if it meant causing some harm to cats? Would you think the same way if you were the parent of such a child? What if you worked at the animal shelter?
Like virtually every other industrialized nation, the United States permits medical experimentation on animals, with few limitations assuming sufficient scientific justification. The goal of any laws that exist is not to ban such tests but rather to limit unnecessary animal suffering by establishing standards for the humane treatment and housing of animals in laboratories. As explained by Stephen Latham, the director of the Interdisciplinary Center for Bioethics at Yale , possible legal and regulatory approaches to animal testing vary on a continuum from strong government regulation and monitoring of all experimentation at one end, to a self-regulated approach that depends on the ethics of the researchers at the other end.
The United Kingdom has the most significant regulatory scheme, whereas Japan uses the self-regulation approach. The U. There is no question that medical research is a valuable and important practice. The question is whether the use of animals is a necessary or even best practice for producing the most reliable results. Other techniques, such as microdosing, use humans not as test animals but as a means to improve the accuracy and reliability of test results.
In vitro methods based on human cell and tissue cultures, stem cells, and genetic testing methods are also increasingly available. The IACUC must include researchers, administrators, a veterinarian, and at least one person with no ties to the institution: that is, a concerned citizen. This committee also performs inspections of laboratories and protocols. As mentioned above, light enters your eyes as a wave. It is important to understand some basic properties of waves to see how they impact what we see.
Two physical characteristics of a wave are amplitude and wavelength Figure 5. The amplitude of a wave is the height of a wave as measured from the highest point on the wave peak or crest to the lowest point on the wave trough.
Wavelength refers to the length of a wave from one peak to the next. Figure 5. The amplitude or height of a wave is measured from the peak to the trough.
The wavelength is measured from peak to peak. In the same manner, the green cone is most sensitive to wavelengths of light associated with the color green. Yet the green cone can also be activated by wavelengths of light associated with the colors yellow and blue.
The graphic below is a sensitivity curve that depicts the range of wavelengths and the sensitivity level for the three kinds of cones. The cone sensitivity curve shown above helps us to better understand our response to the light that is incident upon the retina. While the response is activated by the physics of light waves, the response itself is both physiological and psychological. Suppose that white light - i. Upon striking the retina, the physiological occurs: photochemical reactions occur within the cones to produce electrical impulses that are sent along nerves to the brain.
The cones respond to the incident light by sending a message forward to brain, saying, "Light is hitting me. The brain responds by saying "it is white. And the brain recognizes that the messages are being sent by all three cones and somehow interprets this to mean that white light has entered the eye.
Now suppose that light in the yellow range of wavelengths approximately nm to nm enters the eye and strikes the retina.
Light with these wavelengths would activate both the green and the red cones of the retina. Upon striking the retina, the physiological occurs: electrical messages are sent by both the red and the green cones to the brain. Once received by the brain, the psychological occurs: the brain recognizes that the light has activated both the red and the green cones and somehow interprets this to mean that the object is yellow.
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