Explain how forbidden lines appear in spectra of emission ...

[nramada]

Explain how forbidden lines appear in spectra of emission nebulae.
 
  What will be an ideal response?

Question 2

Pair production can occur if:
 
  A) the energy of two photons is greater than the combined mass-energy of a particle-anti-particle pair.
  B) only virtual particles are produced.
  C) photons are at the event horizon of a black hole.
  D) one particle is struck by a sufficiently high energy photon that a pair of electrons is formed.
  E) the particle and antiparticle have opposite spins.

[nickk12214]

Answer to Question 1

These lines correspond to excited states that require a very long time to decay and emit a photon. In Earth labs, the gas density cannot be made low enough to allow this transition before a collision knocks the atom into another state. In the nebula, though, the density is so low that the transition has a chance to proceed.

Answer to Question 2

A