=== Practice problem 5 ===

Here there are two possible solutions.  Two convergeing lenses can be used with a focal point between them.  Or, a converging lens and a diverging lens with a focal point of the converging lens beyond the diverging lens.

=== Practice problem 8 ===

(d) The relative intensity here must be discussed in terms of the N dependence of the intensity of the principal maximum peak.  (See <<ln(https://youtu.be/zJRSRKcE1do, the later part of this video)>> (the last 10 minutes or so).)

=== Review problem 9 ===

Here, we can use $2 d \sin \phi = m \lambda$.  $d$ is the spacing between lattice planes, and so is not equal to $a_0$.  $\phi$ is not equal to $\theta$.  (See <<ln(https://youtu.be/zJRSRKcE1do, this video)>> (the last 15 minutes).)

=== Review problem 10 ===

(b) This part should be considered a bit out of the scope, for the purpose of this exam, since the change of $f$ due to the change in Snell's law is actually quite complicated.  However, there is a simple additional effect also: when the ray goes through the center, it no longer goes through it as a straight line.  This is also due to Snell's law, and this change is simple to figure out in small angle approximation.  The effect of the second cause alone (assuming that the object does not move, which makes the change of $f$ irrelevant) can be discussed, and the combined effect of these two causes can be discussed also.  The answer becomes ambiguous, even qualitatively, if the combined effect is considered, since the two effects compete.