why do light bulbs explode when in contact with water? Is it true that when water pours on a light bulb it will explode? If so does this apply to all light bulbs and how does that happen.
 A: If the bulb is lit when you pour water on it, it will undergo thermal shock. Some parts cool down and shrink, while other parts are hot. This causes very large thermal stresses, which can break the bulb. Depending on the material and construction of the bulb, this may or may not result in a spectacular implosion (once the bulb cracks, the vacuum inside will be filled with air and the water you poured on the bulb). The water will hit the very hot internal parts, and boil instantaneously. This can make a big mess. Certainly it is not an experiment you should do without extreme precautions.
I used to make CT tubes in a previous life. These are large vacuum tubes with a massive anode that can soak up several MJ of energy (enough for an entire CT scan), at which point they reach a temperature of around 1200 C (meaning they glow "white hot"). One of the safety tests we had to do was to evaluate the behavior of the tube and its enclosure if the vacuum envelope failed and the insulating oil surrounding the tube (which acted both as dielectric insulator and cooling medium) would be sucked into the vacuum, and instantaneously come to a boil against the anode. It was one of the most spectacular experiments I have ever participated in; it was done in a sealed room, with video cameras and other instrumentation all around, in their own sealed boxes. The outermost container of the tube had a baffle that could absorb the expanding oil in normal conditions - in abnormal conditions it would cause boiling oil to spray out, safely directed away from the patient.
Believe me, you don't want to do that experiment unless you really, really know what you are doing.
