while reading the lesson Bromides and Sulphides I passed through this one sentence which was qouted.
If you saw that sunset painted in a picture, you'd never believe it would be possible!
What does this mean ?
Though there be little context to discern from, I will answer as best I can.
The sunset is so outstanding, ethereal, beautiful, resplendent, glorious, or some other such adjective that no one can trust its existence unless he see it with his own eyes. A mere painting is not enough to prove such a thing; one needs to be there and witness it himself to truly believe that something so wondrous can be upon this mortal plane. In short, 'pics or it didn't happen'.
If you provide further context, I may be able to elaborate--but what I have said is probably sufficient.