Final answer:
Rayleigh scattering is the principle that explains why the Earth's sky is blue, as it scatters shorter wavelengths (blue light) more efficiently due to interactions with small gas molecules in the atmosphere.
Step-by-step explanation:
The principle that explains the scattering of shortwave radiation and accounts for Earth's blue sky is known as Rayleigh scattering. Blue light is scattered the most because its wavelength is shorter compared to other visible light colors, causing it to be scattered more efficiently when it encounters the small gas molecules in the Earth's atmosphere. This phenomenon is why the sky appears blue at midday when the sun's position is high in the sky and its path through the atmosphere is relatively short, reducing the amount of blue light reaching the observer directly.