(a) What will be colour of the sky in the absence of atmosphere?
(b) Why are the traffic light signals (or danger signals) of red colour?
(a) In the absence of any atmosphere, there will be no scattering of sunlight and the sky will appear dark.
(b) In the visible spectrum, the red colour has the largest wavelength. The red colour is least scattered by fog or dust particles. Therefore, we can observe red colour easily even in foggy and dusty conditions.