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Q.
A star is going away from the earth. An observer on the earth will see the wavelength of light coming from the star.
Wave Optics
Solution:
Frequency of the light coming from the star moving away from the earth with a speed $v$ will be given by,
$\nu'=\nu \cdot \frac{c}{c+v}$
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.
$\nu' < \nu$
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