Calculate the glancing angle on the cube (100) of a rock salt (a=28.14nm) corresponding to second order diffraction maximum for X-rays of wavelength 14.07 nm.

Options

A. `20^(@)`
B. `30^(@)`
C. `60^(@)`
D. `80^(@)`

Answer

B

Explanation

First determine the distance between lattice plane by using the following expression,
`d_(hkI)=(a)/(sqrt(h^(2)+k^(2)+1^(2)))=2.814`Å, h=1, k=0, 1=0
`therefored_(100)=(2.814Å)/(sqrt(1^(2)+0^(2)+0^(2)))=2.814Å=2.814xx10^(-8)cm`
Using Bragg.s Eqn., `nlamda=2dsintheta`
`2xx2.814xx10^(-8)sintheta=2xx1.407xx10^(-8)`
`sintheta=(2xx1.407xx10^(-8))/(2xx2.814xx10^(-8))=0.5`
`sin30=0.5,theta=30^(@)`.