Drag the yellow dot to explore the wavelength of light emmitted from an HI region and the speed at which the region is moving away from Earth.
When speed = 0 the wavelength is 21 cm
When speed > 0 the region is moving away from Earth and the wavelength is greater than 21 cm
Whens speed < 0 the region is moving towards Earth and the wavelength is less than 21 cm
