Wed February 09, 2011 By:

what is the difference between optical density and mass density?

Expert Reply
Thu February 10, 2011
Dear student,
Material density is "regular" density. Take lead. It is heavy for a small volume of it; it's pretty dense. Water? Not so much. Lead sinks in it. Wood, however, floats on water (at least most kinds of wood). That's material density. Weight per unit of volume of the material. Material density.

Optical density is a term applied to how well light passes through a material. If light is little affected passing through it, that is, if almost all the light goes through it that goes into it, it has a very low optical density. Like clear glass. Very low optical density. Start coloring the glass and less light can get through it; it has a higher optical density than clear glass. The amount of light that can get through a material is addressed in its transmittance, and the amount of light that is "blocked" by a material is addressed in its opacity.

optical density = speed of light / speed in medium

mass density = mass / volume
Hope this helps.
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