Canon's latest Speedlites, the 600EX-RT and the just-announced 430EX III-RT, can be controlled via radio rather than light to avoid the line of sight requirement that older optically-triggered Speedlights have. Does this make them incompatible with the optical system, or do they work with the optical system too?
The 600EX-RT and 430EX III-RT both work with optical triggering and the newer radio triggering system (although you can only use one system at a time). Chapter 5 in the 600EX-RT manual(pdf) covers setting the 600EX-RT up as either master or slave using optical control. There manual for the 430EX III-RT isn't available yet, but the product page tells us:
When used with a compatible flash, or flash driver mounted on camera, the Speedlite 430EX III-RT can serve as a slave unit in an optical setup, at distances of up to 49.2 ft./15m (indoors). In addition to optical wireless flash, the Speedlite 430EX III-RT is capable of radio transmitted wireless flash where the flash units need not be in visual proximity to one another and transmission distance is extended up to 98.4 ft./30m.