Question

A star is moving towards the earth with a speed of $4.5 \times 10^{6}m/s$. If the true wavelength of a certain line in the spectrum received from the star is 5890 Å, its apparent wavelength will be about $\lbrack c = 3 \times 10^{8}m/s\rbrack$

• A. 5890 Å
• B. 5978 Å
• C. 5802 Å
• D. 5896 Å

Answer 1

Answer: (C)

5802 Å

Answer 2

By using $\lambda' = \lambda\left( 1 - \frac{v}{c} \right)$

⇒ $\lambda' = 5890\left( 1 - \frac{4.5 \times 10^{6}}{3 \times 10^{8}} \right) = 5802Å$.