Question

# Give scientific reason:Danger signals are red in colour.

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Hint: The important signals like signals for danger and traffic lights etc. must use a colour which will be visible from a very far way so that any mishap can be avoided. The colour which is used must not fade away with distance. So, if red colour is being used then it means that it fades away the least.

Danger signals are red in colour because the red colour is scattered the least by air, water or dust molecules. The effect of scattering is inversely related to the fourth power of the wavelength of a given colour. Since the colour red has the highest wavelength of all the colours it gets scattered the least. So, red light is used as a danger signal as it is able to travel the longest distance through the fog, rain, etc. without being scattered or faded away. The Rayleigh law states that the scattering of light is inversely proportional to the fourth power of the wavelength of light, which means that the shorter wavelengths in visible white light (violet and blue) are scattered stronger than the longer wavelengths toward the red end of the visible spectrum. When scattering particles are much smaller than the wavelength of light the process is known as Rayleigh scattering.
Scattering $=\dfrac{1}{{{\lambda }^{4}}}$ where, $\lambda$ is the wavelength of the light.

Note: Scattering of light is a phenomenon in which the light rays get deviated from its straight path on striking an obstacle like dust or gas molecules, water vapours etc. The beam of light is redirected in different directions on interacting with the particles present in the medium. This is also the reason behind the appearance of the sky as blue. The sky appears blue because the fine particles in the atmosphere scatter blue light the most among all the components of white light.