The photon has an energy equal to the difference in energy between the orbits. That is why atoms can only emit and absorb photons of specific wavelengths. Observations of emission and absorption lines in a spectrum enable us to determine the composition of celestial bodies. Since every element has a characteristic set of electron orbits, only those photons having certain wavelengths will be emitted or absorbed. This is because the energy of a photon needed to move the electron from one orbit to another is equal to a constant divided by the photon's wavelength.
All wavelengths are present.
An absorption-line spectrum is similar to a continuous spectrum, except that it does have some specific wavelengths darker than surrounding wavelengths. These absorption lines are caused by a cooler gas's atoms absorbing specific wavelengths from the background continuous spectrum. An emission-line spectrum is a spectrum composed entirely of emission lines. Only a few wavelengths are present. An emission line spectrum is seen when astronomers view a hot cloud of rarefied gas with a spectrograph.