Question

The main reason for blueness of the sky is:

• A. Due to the presence of water vapor
• B. Due to the presence of trees
• C. Due to the scattering of sunlight by wind
• D. Due to the scattering of sunlight by air molecules

Hint:

Remember the wavelength-scattering relationship:
- Shorter Wavelengths (Blue/Violet): Scattered most (causes the blue sky during the day).
- Longer Wavelengths (Red/Orange): Scattered least, traveling directly to our eyes when the sun is low on the horizon (causes red suns during sunrise and sunset).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Light from the Sun appears white, but is actually composed of a spectrum of different wavelengths (colors) ranging from red (long wavelength) to violet (short wavelength).
When this light passes through the atmosphere, it interacts with gas molecules and particles.

Step 3: Detailed Explanation:
The blue color of the sky is explained by Rayleigh Scattering:
- Rayleigh scattering describes the scattering of light by particles that are much smaller than the wavelength of the light, such as nitrogen and oxygen molecules in the Earth's atmosphere.
- The intensity of scattered light (\(I\)) is inversely proportional to the fourth power of its wavelength (\(\lambda\)):
\[ I \propto \frac{1}{\lambda^4} \] - Because blue and violet light have much shorter wavelengths than red and orange light, they are scattered in all directions by the gas molecules in the air with much greater intensity (approximately 9.7 times more than red light).
- When we look up, we see this scattered blue light reflecting from gas molecules in all directions.
- Although violet light has an even shorter wavelength and is scattered more than blue, the sky appears blue because our eyes are much more sensitive to blue light, and the Sun emits less violet light compared to blue light.
- Let us evaluate the other options:
- Water vapor (Option A) can cause scattering, but is not the primary cause of the clear blue sky.
- Wind (Option C) and trees (Option B) do not scatter light at a scale that affects the color of the sky.
Therefore, the blue sky is caused by the scattering of sunlight by air molecules.

Step 4: Final Answer:
The blueness of the sky is caused by the scattering of sunlight by air molecules, matching Option (D).