Physics Asked by Oso on February 23, 2021
While cleaning a piece of glass with 70% ethyl alcohol, I made the observation that after swabbing a frosted portion of the glass, the glass appears to lose the "frosted" property temporarily and become almost as transparent as the surrounding glass. What about the act of applying rubbing alcohol produces this effect?
My initial hunch is that since the glass gets the frost effect from creating impurities on the surface by some means such as sandblasting, the alcohol when applied creates a temporarily smooth surface by filling the imperfections to a relatively similar effective height. However, I attempted to recreate the effect with water in place of alcohol and it was significantly less effective, which is the opposite of what I expected given the overall stronger intermolecular forces.
What am I missing? Do I have it backwards and the properties of water like higher surface tension make the effect less pronounced?
Alcohol has a smaller tension surface than water, so when applied it's less likely to form "little bubbles" that works as little lenses and tend to "blur" the light passing trough them (this is caused by the refraction of the light rays changing material). Your assumptions about "filling the holes" is correct. Infact glass and alcohol have pretty similar refractive index, so smoothing the surface gives a smaller "blur effect" because the surface in no more irregular, which when alcohol is not applied cause the scattering of light rays.
Correct answer by lorenzo Baldessarini on February 23, 2021
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