Why do atoms emit energy of only specific wavelengths

There is a difference between the energy of their ground electronic state, where no electrons have been excited by an incoming photon, and their excited electronic state, where at least one electron has been moved to a higher orbital than it normally occupies. The difference between these 2 states determines how much energy they absorb when they get excited and emit when they relax (go back to their ground state). Energy of light absorbed or emitted =Planck's constant*frequency of light absorbed or emitted. Frequency of light = speed/wavelength.

firearm identification is the process of analyzing the bullets and cartridge cases left at a crime scene to determine if they came from a particular firearm. marks on the bullets and cartridges may be common to every firearm of that type. these are called class characteristics.

truck y has a greater velocity than truck x.