That's normal. When you plug it in, there is an inrush of current into the charger. There are filter capacitors inside it that are at 0V before you plug it in, but once you plug it in, they want to get to 120V (or more like 160V for full wave rectified AC) as fast as possible. There is almost always a tiny (imperceptible) spark when you plug anything in. This creates a region of plasma in the air, which is conductive. If you have something on the other end that will draw a lot of power, like an empty capacitor, the conductive plasma spark will move a lot of current, making it bright enough to be visible, and possibly making a pop from the heating of the surrounding air (same idea as lightning and thunder on a tiny scale).
If it keeps sparking after it is plugged in, that's bad and probably means something is loose. If it sparks anywhere other than inside the wall outlet, that's bad too and you might need to have Apple replace the charger.
If you have any outlets protected by an Arc Fault Circuit Interrupter (AFCI), test it there. If it trips the AFCI, there might be a problem, if not, it is probably OK. AFCI is required for all outlets in bedrooms in houses built after 2002. If you are not sure if you have it, look in your electrical panel and see if the circuit breakers for the bedroom circuits say something about AFCI or have an AFCI test button. AFCI is different than GFCI outlets commonly found in kitchens, bathrooms, garages, and outdoors.