# Doppler Shifted Wavelength of Light from Moving HI Region

- Author:
- Wolfe Wall

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