Photos (OS X Yosemite / El Capitan) – show original in Finder?
Photos expects you to export unmodified originals as copies, to preserve the integrity of the internal file management and for collection protection in the same way a database works – it's central to keeping the iCloud storage of your photo collection synced, stable, and functional.

Once you have exported a photo, you can then use mdls and mdfind to see what the "copy" looks like and where the original was stored.

In my case, they are all in ~/Pictures/Photos Library.photoslibrary/Masters/2015/04/08 and such stored by year, month and date for my US locale computer. You'll only have to do this exercise a few times to nail down the internal storage for each time Apple chooses to change it going forward.