Why is it important to select the appropriate wavelength of light when using a spectrophotometer, with the explanation stated in terms of the energy of electromagnetic radiation (EMR)? (HINT: complementary colours).   http://farside.ph.utexas.edu/teaching/302l/lectures/node119.html     http://www.livescience.com/38169-electromagnetism.html http://missionscience.nasa.gov/ems/02_anatomy.html                      

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Electromagnetic radiation (EMR or EM radiation) is all the radiation energy, from gamma rays and X-rays, described as energy, to optical and infrared EM radiation, described as wavelengths, to radio EM radiation, described as frequency.

When using a spectrophotometer, the optical light of the wavelength seen will register as the complement of the wavelength measured because of the absorption properties of wavelengths in the optical range of electromagnetic radiation, or EMR.

As seen in a spectrophotometer, optical electromagnetic radiation wavelengths will be absorbed where the concentration is greatest; this absorbed range of optical EMR will not be seen. This is because what is seen in the spectrophotometer is the EMR that is reflected (not absorbed). What is reflected to the optical range, therefore seen, is the complement of the EMR that is absorbed.

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When using a a spectrophotometer to measure concentration changes in a colored solution you want to select the wavelength at which the greatest absorbance by the solution occurs.

Wavelength of light is related to energy by Planck's constant, h = 6.626 x 10^-34 J-s. The relationship is described by the equation E = h`nu`  where  = frequency or E = (hc)/`lambda` where c = speed of light (3.0 x 10^8 m/s) and `lambda`  = wavelength. 

When we see a substance as a particular color, we're seeing the wavelengths or colors of light that it reflects. Other wavelengths are absorbed. The primary colors of light are red, green and blue. Secondary colors are yellow, magenta and  blue-green.

When a primary color is absorbed, the remaining two primary colors that are reflected make up the complementary secondary color of the absorbed wavelength.

The purpose of the hint is to tell you that the range of maximum absorbance of the substance is most likely the complementary color of the color that the substance appears, not the color that you actually see in the spectrophotometer.

You can find the wavelength that has maximum absorption by starting at the shortest wavelength setting on the spectrophotometer and measuring the absorbance of your solution, then increasing the wavelength setting by 10 nm settings and repeating. 

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A spectrophotometer uses a white light source. All colors of light are present in equal proportions in white light. If any wavelength (color) is removed from the white light, we will perceive the light to be the complementary color of the color which was removed. 

For example, blue and orange are complementary colors. If white light passes through a test tube containing a substance which absorbs blue light, the contents of the test tube will appear orange. 

Thus, when observing a reaction which appears orange to your naked eye, it is important to remember that you should investigate changes in light intensity in the blue wavelengths with the spectrophotometer.

Usually you will be told what wavelength to set the spectrophotometer to. But if you are ever tasked with finding a sensitive wavelength for a particular substance/reaction, be sure to look at the wavelengths which span the complementary color - NOT the color you are seeing with your eyes.

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