Cloud

While the sun expresses his sentiment to the rainbow, a cloud passes by and joins the conversation.

"My dear, you do not need to be disappointed," he tells the sun, "My brother, air, is always around and has been doing as much as you do. Not only does he get no credit, most of the time his existence is even not recognized!"

"Of course! He is so transparent!" The sun and the rainbow laugh and say together.

"Is he?" The cloud makes a curious smile, "Why do you think the air is transparent?"

"What a question!" The rainbow looks puzzled, "I think it is the intrinsic property that air is completely transparent. Isn't it the case?"

"My friend, don't go that far that quickly, "Let me state the question again: Why is air transparent? Is it a question of physics - or a question of biology?"