Light of a wavelength of 661.1 nm is received from a star that is emitting light of a wavelength of 659.6 nm.How fast is the star moving relative to the earth and is it moving toward or away?
A) -580 km/s away
B) 580 km/s away
C) -680 km/s away
D) 680 km/s away

Question

Quizplus · Accepted Answer

The difference in wavelengths is given by the formula:

Δλ/λ = v/c

Where Δλ is the change in wavelength, λ is the original wavelength, v is the velocity of the star and c is the speed of light.

Substituting the given values:

(661.1 - 659.6)/659.6 = v/c

v = (661.1 - 659.6) × (3 × 10^8) / 659.6

v = 1083511.84 m/s or 1083.51 km/s

Since the wavelength has increased, the star is moving away from the Earth. Therefore the velocity is negative.

So, the correct answer is C) -680 km/s away.

Light of a Wavelength of 661