Exam-style question
Try this first
Which statement is true about the speed of light in vacuum?.
- A.It is always 3.00 × 10^8 m/s, independent of the motion of source or observer.
- B.It is 3.00 × 10^8 m/s only when the source is at rest.
- C.It is 3.00 × 10^8 m/s only when the observer is at rest.
- D.It is 3.00 × 10^8 m/s only in a medium with refractive index 1.
Model answer
What a good answer should say
- It is always 3.00 × 10^8 m/s, independent of the motion of source or observer.
Explanation
Why this works
Formula/rule: constancy of the speed of light. Substitution: none.
Working: The second postulate states that c ≈ 3.00 × 10^8 m/s is the same for all inertial observers, regardless of whether the source or observer is moving. Answer: c is constant.
Units/conclusion: c = 3.00 × 10^8 m/s, same in all inertial frames.
Common mistake
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