the AC is run off of your belt. When you turn your ac on the compressor clutch engages and the compressor goes to work. once the engine senses the extra load it compensates and may slightly surge especially on cars with smaller displacement engines. If the engine Surges than the alternator slows briefly causing the lights to dim or blower motor slow. It should just be a very slight surge and its normal. Also because the ac compressor is using the engine to work, it uses horsepower taking away acceleration and UUMFF..