I TA for this student:

On 4/26/07, Karthik.Sivaramakrishnan@asu.edu <Karthik.Sivaramakrishnan@asu.edu > wrote:

It seems you have not turned in your lab report on xrd(lab no. 3)

Reply:
This is true. The reason for not having it turned in is that it was not written. Thank you for the notification, but do not expect to see it unless I hear back from you requesting otherwise.
- Allen

Fermat's principle:

Light follows a path which minimizes the total travel time.

Law of reflection, and law of refraction are a direct fall-out of this!

Courtesy: Applications of Calculus, Philip Straffin

P.S: They chose not to copyright this book! ^:)^