Before Sam explained John the reason for the transparency of air, he asked him about the kind of objects/materials that are transparent? Which of the following may be a plausible reply to the question?
Anything that allows all the light, that falls on it to pass through, is transparent.
An object is said to be transparent when light passes through it without being reflected. If the light falling on an object is absorbed on it, the object will appear dark, if the light is reflected back then the object can be seen but if the light that falls on it is transmitted completely that is all the light is allowed to pass through it then it will appear transparent. Air allows all the light that fall on it to pass through it and hence it is transparent.