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The 6563ÅHalpha line emitted by hydrogen...

The `6563ÅHalpha` line emitted by hydrogen in a star is found to be red shifted by `15Å`. Estimate the speed with which the star is receding from the Earth.

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`lamda=6563xx10^(-10)m, Delta lamda=15xx10^(-10)m, lamda.=lamda+Delta lamda=6578xx10^(-10)m`
`lamda. -lamda=(-v)/c lamda`
`:.v=(-(lamda. -lamda)c)/(lamda)=(-Delta lamda . c)/(lamda)=(-15xx10^(-10)xx3xx10^(8))/(6563xx10^(-10))=-6.86xx10^(5)m//s`
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