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Thread: Light colors and single color light sources

  1. #1 Light colors and single color light sources 
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    From what I know about light, ordinary white light is all colors. Sometimes light can be separated into different colors such as when you use a prism or when you see a rainbow. This is because light of different colors has different wavelengths and light of different wavelengths bends differently. Red has the longest wavelength and so it bends the least and that's why its the outermost and longest and least curved stripe on the rainbow. Violet has the shortest wavelength so it bends the most and that's why its the innermost and most curved stripe on the rainbow.

    Anyway, I was wondering about light sources that produce only one color of light such as the colored lightbulbs on a Christmas tree, or for that matter traffic lights. The red light on a traffic light only emits red light and the green light only emits green light. Same thing with Christmas tree lights, they only emit their respected color. So how do they do that? Are they made with special filaments that only produce light of the wavelength of whatever color they give off?


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    There are two ways, the first is it could be a white light source with a filter which absorbs some of the light between the filament and the glass. This is what is used in traffic lights, they use "white" bulbs but have coloured plastic in front of them to give the desired colour. The second relies on the fact that atoms of different types emit light of different wavelengths when excited. In this case the light contains a vapour of the element that gives the required colour, this is excited by electrical power but when the atoms relax to the ground state they emit the light. The most common examples of this are so called "neon" lights which use the noble gases and yellow/orange street lights that use sodium vapour.


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    Quote Originally Posted by Photon Guy View Post
    Anyway, I was wondering about light sources that produce only one color of light such as the colored lightbulbs on a Christmas tree, or for that matter traffic lights. The red light on a traffic light only emits red light and the green light only emits green light. Same thing with Christmas tree lights, they only emit their respected color. So how do they do that? Are they made with special filaments that only produce light of the wavelength of whatever color they give off?
    Incandescent and fluorescent lights are in the process of being phased out. Taking their place are LEDs, which emit a single wavelength (to an excellent approximation). The color emitted depends on the bandgap of the semiconductor material used to make the LED. The very first LED emitted invisible infrared light, then came red. Other colors followed in due course. With the development of efficient blue and UV LEDs, it became possible to use phosphors (generally yttrium-aluminum-garnet) to produce something that our eyes perceive as white.
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    Quote Originally Posted by tk421 View Post
    Incandescent and fluorescent lights are in the process of being phased out. Taking their place are LEDs, which emit a single wavelength (to an excellent approximation). The color emitted depends on the bandgap of the semiconductor material used to make the LED. The very first LED emitted invisible infrared light, then came red. Other colors followed in due course. With the development of efficient blue and UV LEDs, it became possible to use phosphors (generally yttrium-aluminum-garnet) to produce something that our eyes perceive as white.
    I see. They must make LED flashlights. Im wondering if they make large square shaped LEDs. LED might be the way to go for my project.
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    Quote Originally Posted by Photon Guy View Post
    Quote Originally Posted by tk421 View Post
    Incandescent and fluorescent lights are in the process of being phased out. Taking their place are LEDs, which emit a single wavelength (to an excellent approximation). The color emitted depends on the bandgap of the semiconductor material used to make the LED. The very first LED emitted invisible infrared light, then came red. Other colors followed in due course. With the development of efficient blue and UV LEDs, it became possible to use phosphors (generally yttrium-aluminum-garnet) to produce something that our eyes perceive as white.
    I see. They must make LED flashlights. Im wondering if they make large square shaped LEDs. LED might be the way to go for my project.
    They certainly do make LED flashlights, and LED traffic lights, and backlights for LCDs, and replacements for incandescent bulbs at home and...

    The light-emitting chips themselves are square, but are packaged in a great variety of ways. Just do an online search for "LEDs for sale" or some such thing and you'll be presented with an overwhelming number of options.
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