Banha University

(a) 74 keV (b) 68 keV (c) 87 keV (d) not stated
hf = EM – EK = [- (74 – 9) 2 x 13.6]/9 – [(74) 2 (-13.6/1)] = 68 keV
(4) Which is the following important in the operation of the laser …….
(a)spontaneous emission (b)stimulated emission (c)absorption (d) all what stated
When light is incident on a collection of atoms, a net absorption of energy usually
occurs because when the system is in thermal equilibrium, many more atoms are in
the ground state than in excited states. If the situation can be inverted so that
more atoms are in an excited state than in the ground state, however, a net
emission of photons can result. Such a condition is called population inversion.
Suppose an atom is in the excited E2 and a photon with energy hf = E2 – E1 is
incident on it. The incoming photon can stimulate the excited atom to return to the
ground state and thereby emit a second photon having the same energy hf and
traveling in the same direction. The incident photon is not absorbed, so after the
stimulated emission, there are two identical photons: the incident photon and the
emitted photon. The emitted photon is in phase with the incident photon. These
photons can stimulate other atoms to emit photons in a chain of similar processes.
The many photons produced in this fashion are the source of the intense, coherent
light in a laser.
The first photon that triggers the operation of laser is from the spontaneous
emission process. Once an atom is in an excited state, the atom can make a
transition back to a lower energy level, emitting a photon in the process. This
process is known as spontaneous emission because it happens naturally.
(5) The lattice constant of silicon is 5.43 A. Calculate the volume density of silicon
atoms.
(a) 5x10 22 cm -3 (b) 8x10 22 cm -3 (c) 5x10 25 cm -3 (d) not stated
N = 8/(5.43x10 -8 ) 3 = 5x10 22 cm -3