Physics Problem

Question

A helium-neon laswer produces light with a peak wavelength of 633 nm when at rest. Imagine that such a laser is being carried on a spaceship. If you observe light from the spaceship's laser to have a wavelength of 610 nm, at least how fast is the spaceship moving? Is it moving toward you or away?

Answer 1

This is a Doppler shift problem.  Shorter wavelength means you have a blue shift, so the source is moving toward the observer.  Use the relation

 

Δλ/λ₀ = v/c

 

v = c(Δλ/λ₀)

 

c = speed of light = 3 x 10⁸ m/s

 

Δλ = (633 - 610) x 10^-9 m = ? m

 

λ₀ = 633 x 10^-9 m

 

Plug in the numbers and get v in m/s.