In the hydrogen spectrum,the wavelength of the ${H_\alpha}$ line is $656 \ nm$,whereas in the spectrum of a distant galaxy,the ${H_\alpha}$ line wavelength is $706 \ nm$. The estimated speed of the galaxy with respect to the Earth is:

In the context of the Doppler effect in light,the term 'red shift' signifies:

$A$ star moves away from the Earth at a speed of $0.8\,c$ while emitting light of frequency $6 \times 10^{14} \,Hz$. What frequency will be observed on the Earth? ($c =$ speed of light)

The periodic time of rotation of a certain star is $22$ days and its radius is $7 \times 10^8 \ m$. If the wavelength of light emitted by its surface is $4320 \ \mathring{A}$, the Doppler shift will be $(1 \ day = 86400 \ s)$

The $6563\,\mathring{A}$ $H_{\alpha}$ line emitted by hydrogen in a star is found to be redshifted by $15\,\mathring{A}$. Estimate the speed with which the star is receding from the Earth.

The Doppler effect in sound,in addition to the relative velocity between the source and the observer,also depends on the individual motion of the source and the observer relative to the medium. However,the Doppler effect in light depends only on the relative velocity between the source and the observer. The reason for this is: