Why to use relay for controlling AC light?
AC is alternating current 220v (india) which powers the ac lights. Arduino cannot control high volt n amp, but a relay can do this job, which is the sole design of it. so we are using relay as switch to control high power devices.
What is NO NC and COM in relay?
C = Common Connection
NC = Normally Closed Connection
NO = Normally Open Connection
I have explained NO NC COM elaborately in step 3, refer it -if you feel it bit confusing.
I'm going to use NO (Normally Open Connection).