This section covers the optical effects in air caused by scattering, diffraction, refraction and emission. It also includes anything else which doesn't conveniently fit in the other two categories.
All of these things can be seen without optical aid.
For a few of them, such as aurorae, the images show more than can usually be seen with the eye.
In most cases however it is hard to capture the true effect - you have to see it for yourself.