Are the minions on Despicable Me wearing safety goggles or eyeglasses?
I think you are referring to the detector in a combustion chamber. This is an ultraviolet light detecting device. When it sees ultraviolet light, it knows there is flame. If it sees no ultraviolet, it shuts off the fuel supply. This is to prevent an explosion if fuel hits hot surfaces without there being flame present. Where pilot lights are used, just the light from the pilot is sufficient to keep the sensor happy.
It depends on the fire code in the city you are in (Or is it an OSHA requirement? Everywhere I have worked fire extinguishers are required in public buildings), but I suspect it is and even if it is not, it is a good idea to have one, it may end up saving your life one day.
Energy/volume pressure P Dv^2/2 where D density. P 1000*21^2/2 220500 Pa Note this is the gauge pressure; add 1 atm if the question expects absolute pressure.
