-SaRaH-:
Led is a mere wire at 5v. All we are doing is short circuiting arduino pins.. rite?Yes, if the led has a forward voltage rating of less then or equal to 5vdc, it will turn on and act like a short circuit, just as if you wired an output pin to ground and turned the pin to HIGH. Such a 'short circuit' has no way to limit the output pin current to 40 ma or less and pin damage will soon follow.
At reverse breakdown voltage of LED, it becomes short circuit rite?
Does it still damage the pins when it is reverse biased and voltage exceeds the Breakdown voltage???If the applied reverse voltage is less then it's max reverse voltage limit value then the led will just be turned off and no current will flow and no damage will happen. If the applied reverse voltage exceeds the leds maximum reverse voltage rating then the diode can be damaged and possibly high current can flow.