Helium enters a converging-diverging nozzle with a negligible velocity; stagnation pressure is 400 kPa and stagnation temperature is 400 K. The nozzle throat area is 100 cm2, and the exit area is 600 cm2. Determine the range of nozzle back pressures over which a normal shock will appear in the nozzle. Also, find the nozzle exit velocity if the nozzle exhausts into a vacuum.

absorption lines are where light has been absorbed by the atom thus you see a dip in the spectrum whereas.

emission spectra have spikes in the spectra due to atoms releasing photons at those wavelengths.

explanation:

the order for strong to weak forces such as

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force

the fundamental forces are those forces that act between to objects or element to make interactions. there are four types of fundamental forces such as strong and weak nuclear forces (due to protons and electrons), electromagnetic force (due to the motion of electrons) and gravitational force (due to orbital movement of the planet). if these forces are arranged on the basis of their maximum strength then the order is

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.