a crosses interface of two media you can see the angles will change right or you've had two pulses in two media and we can watch them a race and see which one will fly will travel faster and here you know what I saw a green pulse traveling here it'll excite some fluorescence and this red spot is actually a fluorescent light so we're watching this fluorescent light to decay and with a single laser shot we observed this phenomenon even the force and decay is on the nanosecond scale because we have picosecond resolution so that