I have often wondered this, but never found out...

Why do clouds appear darker when there is sunlight?
As soon as the sun dips behind clouds, the clouds around appear lighter. Is it an optical illusion?

Regards

Not really an optical illusion, but the clouds get lighter where the sun shine through them. When you look at them your eyes adapt to the higher amount of light coming through, therefore the parts that are not letting any direct sun light through seem to be darker. They are not, sometimes because of the spurious light reflected inside the cloud even lighter.

intresting question i just thought more water was being reflected hence the denser shade