Air

The sun and the rainbow are startled.

"You see? Is it the physical property that air is transparent, or is it the biological character of all living creature the perceive that air is transparent?" The cloud explains.

The face of the other two remain blank.

"By physical property, I mean an object is considered transparent if it allows electromagnetic wave of all wavelengths to pass through," the cloud goes on, "As far as I know, the only thing that has this property is vacuum - if vacuum, or nothing, could be considered as something. But, air does not fall into this category. It filters off those with very short wavelengths, such as cosmic X-ray and ultraviolet light."

"Yes...?" The sun murmurs.

"In other words, air is not completely transparent - it only allows the passage of electromagnetic waves whose wavelengths are within the visible spectrum."

"You are just describing the phenomenon by a variety of words. I see no difference there." The rainbow says.