Question

Reason for appearance of red colour of sun during sunrise and sunset is

• A. Scattering of Red light is very small
• B. Scattering of Red light is very high
• C. Sky is blue in colour
  % This is related but not the direct reason for red sun
• D. Muscles in the eye are not working properly % Irrelevant physiological condition

Hint:

- Rayleigh Scattering: Scattering of light by particles much smaller than its wavelength (e.g., air molecules). Intensity of scattering \( \propto 1/\lambda^4 \). - Blue light (shorter \(\lambda\)) is scattered much more than red light (longer \(\lambda\)). - Blue Sky: Due to preferential scattering of blue light from sunlight by the atmosphere in all directions. - Reddish Sun at Sunrise/Sunset: Sunlight travels a longer path through the atmosphere. Most blue light is scattered away, leaving the longer wavelengths (red, orange) to reach the observer directly from the sun.

Answer 1

The Correct Option is A

Step 1: Understand the phenomenon of scattering of light.
Scattering of light is the process by which small particles (like air molecules, dust, water droplets) in the atmosphere deflect light from its straight path.
The amount of scattering depends on the wavelength of light and the size of the scattering particles.
Rayleigh scattering, which applies to particles much smaller than the wavelength of light (like air molecules), is more effective for shorter wavelengths (blue/violet light) than for longer wavelengths (red/orange light).
Specifically, scattering intensity \( \propto 1/\lambda^4 \).

Step 2: Explain the color of the sun at sunrise and sunset.
During sunrise and sunset, sunlight has to travel a longer path through the Earth's atmosphere to reach an observer compared to when the sun is overhead (noon).
- As sunlight passes through this longer atmospheric path, most of the shorter wavelength light (blue, violet) is scattered away from the direct line of sight by air molecules.
- The longer wavelength light (red, orange, yellow) is scattered less and therefore passes through the atmosphere more directly to the observer.
This results in the sun appearing reddish or orange, and the sky around the sun also taking on these hues.

Step 3: Evaluate the options.
Option (1) Scattering of Red light is very small: This is TRUE.
Red light, having a longer wavelength, is scattered much less by air molecules than blue light.
Because other colors are scattered away, red light is what predominantly reaches the observer from the sun.
Option (2) Scattering of Red light is very high: This is FALSE.
Red light is scattered the least among visible colors by air molecules.
Option (3) Sky is blue in colour: This is also due to Rayleigh scattering (blue light scattered more in all directions from sunlight), but it explains why the sky is blue during the day, not directly why the sun appears red at sunrise/sunset, although the same principle is at play.
Option (4) Muscles in the eye are not working properly: This is irrelevant to the atmospheric phenomenon.

Step 4: Identify the most direct reason.
The most direct reason is that red light is scattered the least, so it persists along the direct path from the sun to the observer when sunlight travels through a large amount of atmosphere.
Option (1) correctly states that the scattering of red light is very small.
This matches option (1).