Why are the kilogram, ampere, kelvin, and mole still considered base units and not derived? Given their current definitions, they're purely defined in terms of other units and cannot be independently measured, right?
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I always wondered what happens when light travels through glass in a vacuum - does it lose energy whilst traveling through the glass only to gain it again?
No. The glass will accelerate but that's not because the light slowed down, it's because it became less intense (e.g. some of the photons were stopped by the glass) This is basically how solar sails work