What is Redshift and blueshift? — Physics

Definition 23.13 High School Physics · Chapter 23 — The Doppler Effect

When the spectral lines of a source appear at wavelengths longer than in the laboratory, the spectrum is redshifted; when they appear shorter, it is blueshifted — shifted toward the red or the blue end of the visible spectrum.

$The same hydrogen line in three spectra: at 656.30\, nm in the laboratory, blueshifted for an approaching star, redshifted for a receding one. The shift, read against the dashed reference, gives the radial velocity.$

The same hydrogen line in three spectra: at

656.30\,\mathrm{nm}

in the laboratory, blueshifted for an approaching star, redshifted for a receding one. The shift, read against the dashed reference, gives the radial velocity.

Examples

Example 23.16 (Weighing a star’s escape)

In the spectrum of the bright star Vega, the hydrogen line of laboratory wavelength $656.30\,\mathrm{nm}$ is measured at $656.27\,\mathrm{nm}$ : $\Delta\lambda = -0.03\,\mathrm{nm}$ , a blueshift, so Vega approaches at $v_r = c\,|\Delta\lambda|/\lambda \approx 1.4 \times 10^{4}\,\mathrm{m}/\mathrm{s}$ — $14\,\mathrm{km}/\mathrm{s}$ , read off a shift of one part in twenty thousand.

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Examples

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