Question

The sky appears blue and red (during sunrise and sunset). It is due to the scattering of light by the

• A. dust particles in the atmosphere
• B. water droplets in the atmosphere
• C. ${\mathbf{CO}}_{\mathbf{2}}$ in the atmosphere
• D. none of these

Answer 1

The correct option is A

dust particles in the atmosphere

Step 1: Explanation for the correct option:

(A) Light scattering by the dust particles in the atmosphere:

1. Sunlight is a combination of all colours in the spectrum.
2. When sunlight strikes with particles of size much less than the wavelength of all colours, the light gets scattered. This is known as Rayleigh scattering.
3. The scattering amplitude is inversely proportional to the fourth power of the wavelength of the light.
4. Blue light has the smallest wavelength. Thus it gets scattered the most. And the sky appears blue during the day.
5. During sunrise and sunset, the sunlight travels a longer distance through the atmosphere, thus all the blue components of the light get scattered, and the sky appears reddish.

Step 2: Explanation for the incorrect options:

(B) Light scattering by water droplets in the atmosphere:

1. Mie scattering occurs when the light scatters with particles with a size comparable to the wavelength of the light or larger than the wavelength of light.
2. Unlike Rayleigh scattering, this is a wavelength-independent scattering. Therefore, the lights of all colours scatter equally. And the sky will not appear as blue or red (during sunrise and sunset) as normal if such particles are present in the atmosphere in larger amounts.
3. The water droplets cause Mie scattering.

(C) Light scattering by ${\mathbf{CO}}_{\mathbf{2}}$ in the atmosphere:

1. The size of ${\mathrm{CO}}_2$ is such that it scatters the red side of the light spectrum more than the blue light.
2. So, the presence of ${\mathrm{CO}}_2$ will make the sky appear more orange and red.

Hence, the correct option is (A).