A pencil lead does not emit light when it is attached to the ends of a battery. The lead in a modern pencil is made of graphite. Graphite is a form of carbon and has several properties that make it useful in several applications.
To emit light a substance has to be heated to a very high temperature. The filament used in incandescent lamps is heated to over 2500 degrees Celsius by passing electricity through it. We cannot do the same just by connecting the two ends of a pencil lead to a battery.
A .5mm mechanical pencil lead WILL emit light when enough electrical current is passed through it. I, persoanlly, have tried 5-7 C batteries and it works just fine. It burns quickly and the longer you leave the connection running the brighter it gets, until it burns out, severing the circuit. I have also used a .9mm mechanical pencil lead and 10-12 C batteries. The .9mm mechanical pencil lead became red hot and glowed, but never became hot enough to emit a white light.
Senior Vice President - Phi Theta Kappa AZM
No, pencil lead cannot emit light. Though it is a good conductor, to emit light, an object either has to be red hot or white hot which would require immense amount of heat.