Question

The wavelength of ${\text{H}}_a$ line in hydrogen spectrum was found $6563{\text{A}}^o$ in the laboratory. If the wavelength of same line in the spectrum of a milky way is observed to be $6586{\text{A}}^o$ then the recessional velocity of the milky way will be

• A. $0.105\times {10}^6{\text{ms}}^{-1}$
• B. $1.05\times {10}^6{\text{ms}}^{-1}$
• C. $10.5{\text{ms}}^{-1}$
• D. $\text{none of these}$

Answer 1

The correct option is B $1.05\times {10}^6{\text{ms}}^{-1}$

According to the Doppler effect in light,
$v=\frac{\Delta \lambda \times c}{\lambda }$
$v=\frac{23.0\times {10}^{-10}\times 3.0\times {10}^8}{6563.0\times {10}^{-10}}$
$v=1.051\times {10}^6{\text{ms}}^{-1}$