The blue colour of a clear daytime sky is mainly due to which phenomenon of light?

Introduction / Context:
The colour of the sky is a classic question in basic physics and general science. Understanding why the clear sky appears blue involves the interaction of sunlight with the Earth's atmosphere. This concept links the nature of white light, atmospheric particles, and the scattering of different wavelengths. Competitive exams frequently ask which phenomenon is responsible for the blue colour of the sky, making this a must know topic for students.

Given Data / Assumptions:

• The question asks why the clear sky appears blue.
• Options include scattering, diffraction, dispersion, refraction, and None of the above.
• We assume normal daytime conditions and a clear atmosphere, not sunrise or sunset where colours appear different.

Concept / Approach:
Sunlight is white light composed of different colours with different wavelengths. As this light passes through the Earth's atmosphere, tiny gas molecules and small particles scatter the shorter wavelength components (blue and violet) more strongly than longer wavelengths (red, orange). This wavelength dependent scattering is called Rayleigh scattering. Because our eyes are more sensitive to blue than violet, and some violet is absorbed, the sky appears predominantly blue to us. Thus, scattering of light is the main phenomenon involved.

Step-by-Step Solution:
Step 1: Recognise that sunlight is made of multiple colours, each with its own wavelength. Step 2: Understand that when this light enters the atmosphere, it interacts with gas molecules much smaller than the wavelength of visible light. Step 3: In Rayleigh scattering, shorter wavelengths (blue and violet) are scattered much more strongly than longer wavelengths (red and yellow). Step 4: Due to this preferential scattering, light coming from all directions in the sky is rich in blue components, making the sky look blue. Step 5: Therefore, the correct phenomenon is scattering of light, not diffraction, dispersion, or simple refraction.

Verification / Alternative check:
To verify, think of related phenomena: the reddish colour of the sun at sunrise and sunset is also explained by scattering, where blue light has been scattered away leaving more red light in the direct line of sight. Dispersion explains how white light splits into a spectrum through a prism. Diffraction describes bending of waves around obstacles, and refraction deals with change of direction when light passes between media. Only scattering consistently explains both the blue sky and red sunsets, reinforcing that scattering is the correct choice.

Why Other Options Are Wrong:
Diffraction of light: Involves bending and spreading of light around edges but does not directly cause the sky's blue colour. Dispersion of light: Occurs in prisms and raindrops to form spectra and rainbows, not the everyday blue sky. Refraction of light: Responsible for bending of light in lenses and mirages, not for the overall sky colour. None of the above: Incorrect because scattering of light, listed as an option, accurately describes the phenomenon.

Common Pitfalls:
Students sometimes confuse dispersion and scattering because both involve different colours of light. Another common error is to pick refraction simply because it is a frequently mentioned phenomenon in optics. To avoid such mistakes, remember that dispersion is about splitting light through a medium like a prism, while scattering is about light being redirected in many directions by small particles. The blue colour of the sky is a textbook example of scattering, specifically Rayleigh scattering.

Final Answer:
The blue colour of the clear daytime sky is mainly due to scattering of light in the Earth's atmosphere.