Light from a laboratory source is incident normally at a diffraction grating. The first-order maximum of a spectral line of wavelength 500 nm500\text{ nm}500 nm is observed at an angle of 20.0∘20.0^\circ20.0∘.
Light from a distant receding galaxy is observed using the same diffraction grating with normal incidence. The recession speed of this galaxy is 0.12c0.12c0.12c, where c c\,c is the speed of light in a vacuum.
What is the observed angle of the first-order maximum for this spectral line in the light coming from the galaxy?
17.5∘17.5^\circ17.5∘
22.4∘22.4^\circ22.4∘
22.5∘22.5^\circ22.5∘
25.6∘25.6^\circ25.6∘
