Red is a color at the lowest frequencies of light discernible by the human eye. Red light has a wavelength of roughly 700 nm.
Oxygenated blood is red due to the presence of hemoglobin.
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Lower frequencies are called 
infrared, or 
far red. 
-  Red catches people's attention, and is often seen as a sign of danger: it is thus used for warning signs. 
- The color red is often used to denote anger as in seeing red. 
- The color of blood led red to be associated with the god of war, Mars, and the reddish planet Mars became named for him.  
-  Red is used to denote "stop" in traffic signs and traffic signals.
 
-  In Chinese symbolism, red is the color of good luck and is used for decoration and wedding attire. Money in Chinese societies is traditionally given in red packets.
-  Red is also used to denote debt - as well as losses on a balance sheet.
-  In North American stock markets, red is used to denote a drop in stock   prices. In East Asian stock markets, red is used to denote a rise in stock prices.
 
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