Step 1: Understand the composition of sunlight and the Earth's atmosphere.
Sunlight is white light, composed of various colours (wavelengths). The Earth's atmosphere contains tiny particles such as air molecules (nitrogen and oxygen), water vapour, and dust.
Step 2: Recall the phenomenon of scattering of light.
Scattering of light is the process by which light is deflected in different directions by particles when it encounters them. The amount of scattering depends on the wavelength of light and the size of the scattering particles. Rayleigh scattering, which occurs when the size of the scattering particles is much smaller than the wavelength of light, is strongly dependent on wavelength ( ). Shorter wavelengths are scattered much more strongly than longer wavelengths.
Step 3: Explain why the sky appears blue.
The wavelengths of blue light are shorter than those of other visible colours. When sunlight passes through the Earth's atmosphere, these shorter blue wavelengths are scattered much more effectively in all directions by the tiny air molecules. When we look at the sky, we see this scattered blue light coming from all parts of the sky, making it appear blue.
Step 4: Analyze the other options.
- Dispersion of light:
Dispersion is the splitting of white light into its constituent colours. While the atmosphere can cause some dispersion, it is not the primary reason for the blue colour of the sky. - Refraction of light:
Refraction is the bending of light as it passes from one medium to another. While atmospheric refraction occurs, it does not explain why the sky is blue. - Reflection of light:
Reflection is the bouncing back of light from a surface. While some sunlight is reflected by the atmosphere and the Earth's surface, the blue colour of the sky is not due to reflection.
Therefore, the blue colour of the clear sky is due to the scattering of sunlight by the tiny particles in the Earth's atmosphere, with blue light being scattered more strongly.