Astronomers have found that electromagnetic waves of wavelength 21 cm are continuously reaching the Earth's surface. The frequency of this radiation is . (c = 3×10⁸ m s⁻¹)

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(1) 1.43 GHz. Given: λ = 21 cm = 0.21 m, c = 3×10⁸ m s⁻¹. Idea: 
u = c/λ. The only care needed is to put the wavelength into metres first. 
u = (3×10⁸)/(0.21) = 1.43×10⁹ Hz. 1.43×10⁹ Hz is 1.43 GHz, in the microwave part of the radio band -- which is why it reaches the ground through the atmosphere. This is the famous 21 cm line of atomic hydrogen that radio telescopes use to map the gas in the Galaxy.

Astronomers have found that electromagnetic waves of wavelength 21 cm are continuously reaching the Earth's surface. The frequency of this radiation is ______. (c = 3×10⁸ m s⁻¹)

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