Question

The danger signals installed at the top of tall buildings are red in color. These can be easily seen from a distance because among all other colors, red light

• A. is scattered the most by smoke or fog.
• B. is scattered the least by smoke or fog.
• C. is absorbed the most by smoke or fog.
• D. moves fastest in air.

Hint:

Scattering is inversely related to wavelength:
\[ \text{Wavelength } (\lambda) \uparrow \implies \text{Scattering } \downarrow \]
Red has the largest \(\lambda\) among visible colors, so it scatters the least and travels the farthest.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the scientific reason why danger signals are colored red to ensure they remain visible from long distances even in poor weather conditions like fog or smoke.

Step 2: Key Formula or Approach:
We use Rayleigh's Law of Scattering, which states that the intensity of scattered light (\(I\)) is inversely proportional to the fourth power of its wavelength (\(\lambda\)):
\[ I \propto \frac{1}{\lambda^4} \]

Step 3: Detailed Explanation:

• Visible light consists of a spectrum of colors ranging from violet (shortest wavelength) to red (longest wavelength).

• Since red light has the longest wavelength in the visible spectrum (\(\lambda_{red} \approx 700\text{ nm}\)), according to Rayleigh's scattering law, it experiences the least scattering when interacting with small particles like smoke, dust, or water droplets in fog.

• Shorter wavelengths (like blue and violet) are scattered in all directions and quickly disperse, reducing their intensity along the line of sight.

• Because red light is scattered the least, it can travel through thick fog, smoke, and dust over long distances without significant loss of intensity.

• This allows the danger signal to remain highly visible and sharp from a long distance.


Step 4: Final Answer:
Red light is scattered the least by smoke or fog.