Question

For an observer on the earth, if a spectral line of wavelength $6600~\mathring{A$ emitted by a star is found to be redshifted by $22~\mathring{A}$, then the star is}

• A. Receding away with $9 \times 10^5~\text{m/s}$
• B. Receding away with $10^6~\text{m/s}$
• C. Moving towards earth with $9 \times 10^5~\text{m/s}$
• D. Moving towards earth with $10^6~\text{m/s}$

Hint:

Redshift implies star is moving away. Use $\dfrac{\Delta \lambda}{\lambda} = \dfrac{v}{c}$.

Answer 1

The Correct Option is B

Redshift formula: $\dfrac{\Delta \lambda}{\lambda} = \dfrac{v}{c}$
$\dfrac{22}{6600} = \dfrac{v}{3 \times 10^8}$
$v = \dfrac{22}{6600} \cdot 3 \times 10^8 = \dfrac{1}{300} \cdot 3 \times 10^8 = 10^6~\text{m/s}$
Since redshifted, star is receding.