You observe a x-ray line with a wavelength of 0.144 nm.
a. what is the energy of the x-ray photon, in ev?
b. determine which element (name and atomic number) produced the line.
c. if the element found in part (b) was modeled as a bohr atom (that is, with only one electron but the same atomic number), what
would be the energy of the transition of the transition from the n=2 to n=1 state? how does this value compare with your value in
part (a)?

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me