If the negative is on the positive side of the circuit no current will flow. The most important part of any LED is the semi-conductor chip, which is located in the center of the lens. The chip has two regions separated by a special junction. One region inside the lens is dominated by positive electric charges, and the other region is dominated by negative electric charges. The junction acts as a barrier to the flow of electrons between these regions.
Any material can emit only photons in a very narrow frequency range.
LED's that emit different colors are made of different semi-conductor materials, and require different energies to light them. All rights reserved.