(5-1)   ①  Write down an expression, in terms of the plane separation, d, and the angle θ,  for the extra distance covered by a ray emerging from the crystal having travelled path A, compared to path B.


② Construct a general expression for the condition which must be met to get an intensity maximum in the reflected wave. This should be a relationship between the plane separation (d), the angle between the incident x-rays and the crystal surface (θ), and the x-ray wavelength (λ).


③  What is the condition for an intensity minimum?