In atmospheric optics, the clear daytime sky appears blue mainly because of which of the following physical processes acting on sunlight?

Introduction / Context:
One of the most familiar questions in basic physics and everyday life is why the sky looks blue on a clear day. Understanding this requires some knowledge of how sunlight interacts with the Earth atmosphere. The correct explanation involves scattering of light, and recognising this phenomenon is a common test of conceptual understanding in school level optics.

Given Data / Assumptions:

• The phenomenon described is the blue colour of a clear daytime sky.

• Several possible optical processes are listed: scattering, dispersion, reflection, refraction and absorption.

• It is assumed that the sky is clear, without heavy clouds or rain.

• The learner is expected to know the basic idea of wavelength dependent scattering.

Concept / Approach:
Sunlight is composed of many colours. When it enters the Earth atmosphere, the small molecules of gases such as nitrogen and oxygen scatter shorter wavelength light (blue and violet) more strongly than longer wavelength light (red and orange). This wavelength dependent scattering is called Rayleigh scattering. Our eyes are more sensitive to blue than violet, and some violet is absorbed, so the sky appears predominantly blue. Therefore, the correct description is scattering of sunlight by air molecules and fine particles, not dispersion, reflection or refraction alone.

Step-by-Step Solution:
Step 1: Recall that white sunlight contains a spectrum of colours from violet to red. Step 2: Understand that when sunlight passes through the atmosphere, it interacts with gas molecules and tiny particles. Step 3: Recognise that shorter wavelengths (blue and violet) are scattered much more strongly than longer wavelengths. Step 4: Note that scattered blue light reaches our eyes from all directions, making the sky appear blue. Step 5: Among the options, select scattering of sunlight by molecules and fine dust in the atmosphere as the process that matches this explanation.

Verification / Alternative check:
Physics textbooks that discuss Rayleigh scattering explicitly connect it with the blue colour of the sky and the reddish colour of the Sun at sunrise and sunset. Diagrams show sunlight entering the atmosphere and being scattered, with blue rays more widely distributed. In contrast, dispersion by raindrops is used to explain rainbows, and reflection from clouds explains cloud brightness, not clear sky colour. Refraction is involved in bending of light but does not by itself colour the sky. This standard treatment clearly supports scattering as the correct process.

Why Other Options Are Wrong:
Dispersion of light by raindrops forms colourful rainbows, which appear only when rain and sunlight occur together, not when the sky is uniformly clear and blue. Reflection of sunlight from clouds explains why clouds look white or grey, but the question specifically mentions a clear sky without cloud cover. Refraction of light by air layers causes effects like mirages but does not alone create the blue colour of the sky. Absorption by the ozone layer primarily affects ultraviolet light and does not selectively leave only blue visible light. Therefore, these alternatives do not correctly account for the observed phenomenon.

Common Pitfalls:
Students sometimes confuse dispersion and scattering, because both involve separation of colours. Another pitfall is to focus on any familiar optical term without considering the exact conditions described in the question. The safest strategy is to associate blue sky with Rayleigh scattering of shorter wavelengths and rainbows with dispersion in raindrops. Once this mapping is clear, selecting the correct option becomes straightforward.

Final Answer:
The clear daytime sky appears blue mainly due to scattering of sunlight by molecules and fine dust in the atmosphere.