The Human Eye

			Visible Light	A Rainbow

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	The human eye "visible light" spectrum is between 400nm to 700nm the tiny piece of the spectrum between Ultraviolet and Infrared
	A rainbow shows the "Visible light" spectrum
	Different colors correspond to different wavelengths
	Long wavelengths (right side of chart above) are low frequency
	Short wavelengths (left side of chart above) are high frequency
	"Visible light" colors have different wavelengths, frequencies, and energies
	Violet has the shortest wavelength in the visible spectrum. That means it has the highest frequency and energy. Red has the longest wavelength, and lowest frequency and energy.
	measurement used for wavelengths is nanometers (nm)
	Infrared lights used on investigations show up as red because they are usually between 650nm - 820nm
	Light from the sun or a light bulb may look white, but it is actually a combination of many colors. We can see the different colors of the spectrum by splitting the light with a prism.
	Blue wavelengths are shorter in the visible spectrum, they are scattered more easily in the atmosphere. Our eyes are more sensitive to blue light than violet. Also the sun emits more light as blue than violet. Thus the sky appears blue.
	Grass appears green because all of the other colors of the "visible light" spectrum are absorbed into the leaves of grass except green. Green is reflected so the grass appears green.

 

			Human Eye	Cones & Rods	Visual Field

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	Images are formed by the retina
	The retina contains two types of cells: rods and cones
	Cones measure color (red, green, blue) : color vision, daylight levels. Cones are non-functional in low light. Function best in bright light.  Cone cells are sparser towards the periphery of the retina.
	Rods are responsible for monochrome night-vision : grayscale, low light levels, scotopic (monochrome) vision  and peripheral vision
	Peripheral vision of complex shapes and colors is poor. Details only seen when looking right at an image or object.
	Read this sentence and concentrate on one word. How many other words can you see clearly while still concentrating on one word? Not many. This is an example of how limited human vision can be.
	Using peripheral vision it is possible to see faces and figures in random patterns. The human brain attempts to identify familiar patterns in random patterns.
	Rods are more sensitive to light and movement than cones but cannot see colors.
	This could help explain some instances of the sightings of  shadows or "shadow people"  out of the "corner of the eye".  Turning and looking directly at the object or image, it is sometimes said to "vanish".
	Note that what is listed above does not explain every sighting of so called "shadow people" but it could be a reasonably good explanation for some of the occurrences.
	References:
	http://en.wikipedia.org/wiki/Eye#Rods_and_cones
	http://en.wikipedia.org/wiki/Cone_cell
	http://en.wikipedia.org/wiki/Rod_cell
	http://webvision.med.utah.edu/sretina.html
	http://nn.cs.utexas.edu/web-pubs/bednar.thesis/node6.html