Why is red light employed for danger signals?

Red light is used for danger signals primarily because it is scattered the least by atmospheric particles like fog, smoke, or dust [1][t3]. According to Rayleigh's law of scattering, the intensity of scattering is inversely proportional to the fourth power of the wavelength [t5]. Since red light has the longest wavelength in the visible spectrum, it undergoes minimal scattering compared to shorter wavelengths like blue or violet [t2][t3]. This property allows red light to travel longer distances through the atmosphere without losing its intensity or changing its perceived color, ensuring it remains visible to observers from a great distance even in adverse weather conditions [1][t5]. While the human eye is sensitive to various colors, it is actually most sensitive to yellow-green wavelengths (~555 nm) in bright light, making scattering the definitive physical reason for choosing red for long-range signaling [t6][t10].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 10: The Human Eye and the Colourful World > 10.6.2 Why is the colour of the clear Sky Blue? > p. 169