Question

Red light is used for signals because it has

• A. long wavelength
• B. high intensity
• C. high frequency
• D. low refraction in the medium

Hint:

Red light is used in danger signals because it has the longest wavelength and undergoes the least scattering.

Answer 1

The Correct Option is A

Concept:
Red light has the longest wavelength among visible colours. Scattering of light is inversely related to the fourth power of wavelength: \[ \text{Scattering} \propto \frac{1}{\lambda^4} \] where, \[ \lambda=\text{wavelength} \] So, light with longer wavelength scatters less.

Step 1: Recall wavelength of red light.
Among visible colours: \[ \lambda_{\text{red}} > \lambda_{\text{orange}} > \lambda_{\text{yellow}} > \lambda_{\text{green}} > \lambda_{\text{blue}} > \lambda_{\text{violet}} \] So red light has maximum wavelength in the visible spectrum.

Step 2: Understand why less scattering is useful.
In air, fog, dust, or smoke, light can scatter. If light scatters too much, it cannot travel long distances clearly. Since red light has a long wavelength, it scatters least. Therefore, it can be seen from far away.

Step 3: Application in signals.
Traffic signals, danger signals, and railway signals use red light because it remains visible over long distances. This is very useful for warning and safety.

Step 4: Check the options.
Option (A) long wavelength is correct.
Option (B) high intensity is not the main reason.
Option (C) high frequency is incorrect because red light has lower frequency than blue or violet.
Option (D) low refraction is not the main reason for using red light in signals. Hence, the correct answer is: \[ \boxed{(A)\ \text{long wavelength}} \]