Question

You observe a distant galaxy. You find that a spectral line, resulting from an electron transition in hydrogen, is shifted from its normal location in the visible part of the spectrum into the infrared part of the spectrum. What can you conclude?

Answer 1

The galaxy is moving away from the observer

Explanation: when a galaxy is moving away from us, the light we percieve from it is "streched". Since the wavelength has an inverse raltionship whith frequency, the longer the wavelength is, the lower the frequency. And lower frequencies correspond to red and infrarred light.

So when we see the light has shifted to the infrarred part of the spectrum, it means the source is traveling away from us, making the light waves we percieve streched and move from visible light to infrarred.