A servo is a device that rotates 180 degrees max, you can program it to alter degrees smaller than 180 degrees. The servo points whether the light is on or off on paper, which the LDR/photoresistor will detect and give us values, which we could determine whether or not the light is on or off and making the servo rotate that way.
When my study lamp was on, the arrow pointed right.
When my study lamp was off, the arrow pointed left.