If the op-amp is used without feedback it has a very high gain but is unstable, and its outputs swings very quickly between the voltage rails. In this way it can be used to produce a digital voltage. The driving voltages for the op-amp can be chosen.
Typical values: +15V to –15V, +9 to –9, 5V to 0V. The outputs will not usually exceed a saturation value – typically 13.5V when driven with +15V.

The op-amp compares the 2 inputs, and switches the output (to high or low )

If V1 is bigger than V2 then the output is low.
If V1 is smaller then the output is high.

V- is held at 1/(10+1)*6 = 0.55V. The second potential divider circuit includes a LDR so that V+ varies with the light shining. In the Dark resistance of LDR is very high, so V+ is very high, so it drives the op-amp to its positive rail. So turning on the transistor and the ammeter shows a reading. Output device is ON.