What gives color to colloids?

 

 

 

 

 

 

 

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Silver Nanoparticles: Optical Properties
Silver nanoparticles absorb and scatter light with extraordinary efficiency. Their strong interaction with light occurs because the conduction electrons on the metal surface undergo a collective oscillation when they are excited by light at specific wavelengths. This oscillation is known as a surface plasmon resonance (SPR), and it causes the absorption and scattering intensities of silver nanoparticles to be much higher than identically sized non-plasmonic nanoparticles. Silver nanoparticle absorption and scattering properties can be tuned by controlling the particle size, shape, and the local refractive index near the particle surface.

 

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https://en.wikipedia.org/wiki/Colloidal_gold

Colloidal gold is a sol or colloidal suspension of nanoparticles of gold in a fluid, usually water. The colloid is usually either an intense red colour (for spherical particles less than 100 nm) or blue/purple (for larger spherical particles or nanorods).[1] Due to their optical, electronic, and molecular-recognition properties, gold nanoparticles are the subject of substantial research, with many potential or promised applications in a wide variety of areas, including electron microscopyelectronicsnanotechnologymaterials science, and biomedicine.