Question

Multi-part question If a galaxy moving away from the Earth has a speed of 1000 km/s and emits 656 nm light characteristic of hydrogen (the most common element in the universe). What wavelength would we observe on the Earth

Answer 1

Answer:

658.2 nm

Explanation:

Since the galaxy is moving at relavitistic speed, we use the equation for relativistic Doppler shift of light.

So, the wavelength of light observed on the Earth is λ

λ = λ'($\sqrt{\frac{ 1 + \frac{v}{c} }{1 - \frac{v}{c} } }$)

where  λ' = wavelength of light emitted by galaxy = 656 nm, v = speed of galaxy = 1000 km/s (positive since the galaxy is moving away from the Earth) and c = speed of light = 300000 km/s

So, substituting the values of the variables into the equation, we have

λ = λ'(√[{1 + (v/c)}/(1 - (v/c)]

λ = 656 nm(√[{1 + (1000 km/s/300000 km/s)}/(1 - (1000 km/s/300000 km/s)]

λ = 656 nm(√[{1 + 1/300}/(1 - 1/300]

λ = 656 nm(√[{(300 + 1)/300}/{(300 - 1)/300}]

λ = 656 nm(√[{(301)/300}/{(299)/300}]

λ = 656 nm(√[301/299])

λ = 656 nm(√1.0067)

λ = 656 nm × 1.0033

λ = 658.19 nm

λ ≅ 658.2 nm

So, the wavelength observed on Earth is 658.2 nm