Enhance Rainbows with a CPL filter
Rainbows are somewhat of a cliché in landscape photography. However, it is undeniable how they can add interest to the scene that would otherwise be drab and perhaps boring.
Sometimes in photos they will look washed out and uninteresting- a poor representation of what you can see before you. Thankfully, there is a very easy way this can be remedied. This is through use of a circular polarising filter (CPL).
You might think that as a CPL is usually used to cut out polarised light and that rainbows consist of some degree of polarised light that using a CPL to photograph rainbows is a bad idea. However if you've never tried experimenting with CPLs and rainbow photography then next time you see one give it a go.
You will find that the intensity of colours in the rainbow is directly affected by the polariser's rotation. If you rotate the polariser you may find that the rainbow entirely disappears; this isn't what we want in this particular situation. But if you keep rotating the polariser, you will find the cause of the rainbow will become more intense and much brighter.
This is because when light reflects of droplets of water to produce the rainbow it is polarised in more than one direction (or plane). By using a CPL it is easy to enhance one plane of polarisation by cutting out another… making for a more brilliant photograph of the rainbow. That's all there is to it. You don't really need to understand the physics though- just know that it works! Next time you're out and about and see a rainbow quickly screw on a CPL and try for yourself!
Just as an example, here is a photograph first taken without a CPL and then another very similar shot taken immediately after with a polariser. As you can see the rainbow is much more intense and much more visible. Both these photos are unedited and straight from camera.
Now, with some very simple edits in Adobe white room, it is easy to get a much more vibrant and interesting photograph as you can see below: