Well, Light is just Photons that are moving. Photons are an elementary particle and since they have absolutely no mass they can travel the fastest. The sun burns it's own mass witch is you guessed it, Hydrogen to generate very hot energy called plasma through the process of nuclear fusion. When the Dragon digests food the Hydrogen and Methane byproduct is transported to a extremely special organ that excretes a mucus that is composed of mainly lead. When the Dragon munches on some of the very abundant source of radioactive ore that had plagued the earth in that time period, the ore is stored in that organ. As the radioactive ore decays, the energy produced would be powerful enough to rip the Hydrogen and Methane atoms apart and fuse them together to form other elements. this process in a very simple way would be defined as a Feedback Loop(Some of the energy produced would feed the production of more energy and so on). the Lead lining would not only protect the Dragon's body from the radioactivity, but also would contain the energy being produced. As the energy builds up, the ore is quickly torn apart and once the heat is high enough the Hydrogen and Methane that was aiding in the production of the energy, would ignite at an incredible temperature. That would then create plasma(Witch is Toothles's fire blast), that plasma would be hot enough to "Throw" Protons around. Then all the Dragon needs to do is direct the insane amount of Photons that are being produced in one direction, luckily Photons(AKA: Light) don't like to change direction. So as the Dragon attacks the amount of light being channeled through it's throat and out the mouth would not only blind anything on a scale compared to the biggest detonation in human history(tsar nuclear bomb, but times ten), but also incinerate anything and I mean anything down to it's respective elemental particles, like Protons, Neutrons, and Electrons. you may ask "But doesn't Uranium and other radioactive elements take hundreds of millions of years to decay completely?" you are correct but the process of the Hydrogen and Methane combining through Fusion while the Uranium decays through Fission generates so much energy that the decaying process is sped up insanely fast. For an example: Nuclear reactors operate by shooting Neutrons at Uranium that causes instability in the atoms. The resulting action generates energy. The more Neutrons are shot at the Uranium, the faster it decays.
