With what speed should a star move with respect to us so that the beam at wavelength 460.0 nm is observed at 460.8 nm.

Explanation

Now we have , `(Delta v)/(v) = -(Deltalambda)/(lambda)=-(-v_("radial"))/(c)`
Now , `Delta lambda , (460.8-460.0)nm = 0. 8 nm`
Now , `v_("radial")=(cDelta lambda)/(lambda)`
`=(3 xx 10^(8)xx 0.8nm)/(460nm)`
`= 5.217 xx 10^(5) m//s`
= 521 . 7 km /s
Since, v comes out to be positive, hence we can conclude that the star is moving away from us .