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Why is the speed of light in a vacuum treated as one of the basic fixed physical quantities?
AIt changes slightly depending on which direction light travels
BIt is precisely known and the same in every vacuum
CIt is only approximately known, unlike other constants
DIt is defined separately for each colour of light
Answer & Solution
Correct answer: B. It is precisely known and the same in every vacuum
1. The speed of light in a vacuum is described as so important and so precisely known that it is treated as a basic physical quantity.
2. This fixed value, c, is the same for every colour and every direction in a vacuum.
3. An option that makes c vary by colour or direction contradicts the idea of a single fixed vacuum speed.
_Source: OpenStax Physics (CC BY 4.0), Ch 16 "Mirrors and Lenses", section 16.2 Refraction_
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