What will happen when 3 1.5 volt batteries are connected to an LED?

When three 1.5-volt batteries are connected in series, they will provide a total voltage of 4.5 volts to the LED. If the LED's forward voltage rating is lower than 4.5 volts (typically around 2-3 volts for standard LEDs), it will illuminate brightly. However, if the voltage exceeds the LED's rating, it could lead to overheating and damage the LED. To ensure safe operation, a current-limiting resistor is often used in series with the LED.