Week 7, clip 2

Timepoint:  09:21.9

1) What wavelength is used to measure the presence of neutral, atomic hydrogen (HI)?

- 210 nm (2.1e-7 meter); in the ultraviolet 
- 2.1 $$\mu$$m (2.1e-6 meter); in the infrared
- 21 cm (2.1e-2 meter); in the radio
- 2.1 meter; in the radio

Answers:
1) 21 cm (2.1e-2 meter); in the radio