Question

Give reasons:  
(i) The sky appears dark to passengers flying at very high altitudes. 
At very high altitudes, passengers are above the atmosphere where there is less scattering of sunlight. As a result, they do not see the scattered blue light and the sky appears dark, similar to the condition experienced by astronauts in space. 
(ii) 'Danger' signal lights are red in color. 

Hint:

The refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium. The bending of light depends on the relative speed in different media.

Answer 1

Due to its longest wavelength and minimal atmospheric scattering, red light is observable from greater distances. Consequently, it's employed in 'Danger' signals.