The danger signal lights are usually red in colour because red is:

Question

Examoogle · Accepted Answer

The correct answer is Option B: least scattered by fog or smoke. Danger signal lights are red because red light has the longest wavelength in the visible spectrum. Consequently, it undergoes the least scattering by atmospheric particles like fog, smoke, or dust, allowing the light to travel over longer distances and be seen clearly by observers.

Step-by-Step Solution & Concepts:
Danger signal lights are red because red light has the longest wavelength in the visible spectrum. Consequently, it undergoes the least scattering by atmospheric particles like fog, smoke, or dust, allowing the light to travel over longer distances and be seen clearly by observers.

Working Principle: According to Rayleigh's law of scattering, the intensity of scattered light $I$ is inversely proportional to the fourth power of its wavelength $\lambda$ ($I \propto \frac{1}{\lambda^4}$). Since red light has the longest wavelength among visible colors, it is scattered the least by particles whose size is comparable to or smaller than the wavelength of visible light.

Key Formulas: $I \propto \frac{1}{\lambda^4}$ — Rayleigh scattering law where $I$ is scattered intensity and $\lambda$ is wavelength.

Key Points: Red light has the longest wavelength (~620–750 nm) in the visible spectrum.; Rayleigh scattering explains why the sky is blue (shorter wavelengths scatter more) and why sunsets and danger signals appear red (longer wavelengths survive scattering).; Visibility of danger signals is enhanced because red light penetrates through obscuring media like fog or mist effectively.

Advantages: High visibility over long distances.; Red color is biologically distinct and universally associated with urgency or danger.

Applications: Railway and road traffic signal lights.; Navigation lights on ships and aircraft.; Warning lights on high-rise structures (aviation obstruction lights).

Additional Info: Violet and blue light, having shorter wavelengths, are scattered much more significantly than red light.. Option A is incorrect because scattering is minimal for red, not reflection; Option C is incorrect as scattering is physically present but minimal; Option D is incorrect as it describes the properties of blue/violet light.

Answer: B is correct — Red light is least scattered by fog or smoke due to its long wavelength, making it highly visible over long distances.

Exam Tip: Always remember that scattering is inversely proportional to the 4th power of wavelength. Questions about the color of the sky, danger signals, and the redness of the sun at sunrise/sunset all rely on this single principle.

Examoogle · Answer

Option A: highest reflected by fog or smoke

Examoogle · Answer

Option C: not scattered at all by fog or smoke

Examoogle · Answer

Option D: highest scattered by fog or smoke

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