How to design a power switching circuit using a photo-interrupter
It's not a great circuit. The PNP transistor would not switch on fully so I'd use an NPN with the emitter grounded - this way, current into the base turns the transistor on to about 0.1 volt across collector and emitter and wastes less power.

Having said all that I'd use an N channel MOSFET that maybe switches on to a few millivolts.