A source at rest emits light of wavelength 500 nm. When it is moving at 0.90c away from an observer, the observer detects light of wavelength:
A) 26 nm
B) 115 nm
C) 500 nm
D) 2200 nm
E) 9500 nm

Question

Quizplus · Accepted Answer

The observed increase in wavelength due to the source moving away from the observer can be calculated using the Doppler effect formula for light. The formula for the observed wavelength ($\lambda'$) when the source is moving away is $\lambda' = \lambda \sqrt{\frac{1 + \beta}{1 - \beta}}$, where $\lambda$ is the original wavelength (500 nm) and $\beta = v/c$ (with $v = 0.90c$, so $\beta = 0.90$). Plugging in the values, we find that the observed wavelength is significantly longer than the original, which matches option D, 2200 nm.

A Source at Rest Emits Light of Wavelength 500 Nm