Like any other diode, an LED carries current only in one direction. Explain why an LED's brightness is proportional to the electric current flowing through it. As more current flows through the LED, more electrons make the shift and emit photons of light. The current passing through an LED experiences a voltage drop. Use conservation of energy to explain why this voltage drop must occur.
Light carries away energy.
If the current passing through the LED experiences zero voltage drop, it doesn't leave any energy in the LED and the LED can't produce a steady output of light. If the LED is steadily emitting light and light carries energy, then the LED needs a steady source of energy. The electric current passing through the LED must leave energy in the LED and the result of such an energy delivery is a voltage drop for the current. After all, a drop in voltage indicates that the current has lost energy per charge. A photon of green light has more energy than a photon of red light.