Question

A star is going away from the earth. An observer on the earth will see the wavelength of light coming from the star.

• A. decreased
• B. increased
• C. neither decreased nor increased
• D. decreased or increased depending upon the velocity of the star.

Answer 1

Answer: (B)

Frequency of the light coming from the star moving away from the earth with a speed $v$ will be given by,
${\nu }^{\prime }=\nu \cdot \frac{c}{c+v}$
where, $8=$ actual frequency
$c=$ velocity of light
$v=$ velocity of the star
Now, if star is going away then from equation (i) we can say.
${\nu }^{\prime }<\nu$
Hence, observer on the earth will see a light of decreased frequency. and we know,
$\rho =\frac{C}{\lambda }$
$[$ where, $\lambda =$ wavelength $]$
then, wavelength will increase in this case. and the observere on earth will see the wavelength of light