(a) Give reasons: (i) The sky appears dark to passengers flying at very high altitude. (ii) 'Danger' signal lights are red in colour. OR (b) What is a rainbow? "We see a rainbow in the sky only after the rainfall." Why?

(a) (i) At very high altitude, the atmosphere is too thin to scatter the sunlight. As a result, the sky appears dark to passengers flying at very high altitude. (ii) Red colour has the maximum wavelength, so it scatters the least. Hence, 'Danger' signal lights are red in colour. <br> (b) A rainbow is a beautiful natural phenomenon in which different colours of light appear in the sky when sunlight splits through the water droplets in the air. We see a rainbow in the sky only after the rainfall because at this time, the air has enough water droplets to create a rainbow.