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Learn more about Color of Light Absorbed

We see the world through light. Light is comprised of photons that travel at different wavelengths and energies. The wavelength of visible light can be seen between 350 and 750 nm. White light comprises the full visible spectrum: violet, indigo, blue, green, yellow, orange and red. 

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Every object will either transmit, absorb or reflect light. If an object fully transmits all light, it will be colorless and transparent; if it absorbs all visible light, it will be black and opaque; if it reflects all visible light, it will be white and opaque. By altering the transmission, absorption and reflection of objects, we can change its color properties.

This table will match the "colored of light absorbed" with "observed color of a part". The color we see of an object is not the color it absorbs but the light it reflects. Therefore, if we want an object to look purple, we will need to absorb a large portion of yellow light.

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After a dye absorbs light, some dyes can then emit light. This process is called emission and can be used in a variety of applications including labeling and sensing. When a dye emits light, it will typically emit the light at a higher wavelength. This is due to a loss of energy during the absorption and emission of light from a dye.  

How well a material absorbs light is called its molar absorptivity, typically given the symbol ɛ. When an absorbing material is dissolved in a solvent, the absorbance is measured using a UV/Vis spectrometer. The Absorbance, A, is related to the distinct molar absorptivity of the material, ɛ, the concentration of the material in the solvent, C, and the pathlength the measuring light traveled through, l. With that we get the equation:

A=ɛ*C*l

The value ɛ is typically given at a specific wavelength known as the ʎ max. The ʎ max can vary depending on the phase of the absorbing material, the particle size of the material or the solvent the material is dissolved in. For example, if a material absorbs at 500 nm when dissolved in acetone, the same material in plastic will likely absorb between 515 and 520 nm. The difference from ʎ max in acetone to the ʎ max in plastic is called a redshift. If the ʎ max shifts to a lower wavelength, such as from 520 nm to 500 nm, it is called a blue shift. These terms come from the higher wavelengths of visible light ending in red and lower wavelengths of visible light are blue.