|MadSci Network: Physics|
Hello James, For all practical purposes, a photocell does respond to all wavelengths of light; light being but a small part of the electromagnectic spectrum. What makes a photocell create electron flow is its' constituent parts - the materials that have electroluminescent properties. If we look at a simple Light Emitting Diode (LED), we can see that the wavelength of light created is a function of the materials it is "doped" with. Some of the more common materials are Ga, Ar, Se, In, Al, Ge, N, and P. You can look them up in your periodic table of the elements for their full names. By analyzing the output curves of a particular device, you will see that it is a bell-shaped curve, with the maximum light output at the wavelength determined by its' chemical composition. Other frequencies of light are produced, but drop off rapidly only a few nanometers on either side of the peak frequency. Here is a link to a cyan (blue) led from Agilent Technologies; a division of Hewlitt Packard, which illustrates this. Note: You will need Acrobat Reader to view the datasheet. http://literature.agilent.com/litweb/pdf/5968-7193E.pdf In theory, it would be possible to create a photocell that is responsive to all wavelengths of light, but the practicality of it is quite another matter. To achieve an ideal photocell, i.e. the same output (mW/cm2) regardless of the wavelength of light incident upon it, would be a daunting task; to say the least. What's more, the sun, being the most abundant light source, is made up of all wavelengths of light - but in varying proportions. As the sun changes position in the sky, the proportions of "white" light to that of infrared and ultraviolet change dramatically due primarily to atmospheric absorption of certain wavelengths. Even the human eye is not perfect. We cannot "see" infrared or ultraviolet at all (some insects can see ultraviolet), and our eye's sensitivity to all wavelengths of visible light is not equal. For example, our eyes are more sensitive to green light than to red. The following link is to an interesting article on the Photon Belt. http://www.salemctr.com /photon/center5c.html Your not-so-mad scientist, Karl Kolbus KarlKolbus@ameritech.net
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