//example-5.1
//page no-136
//given
//wavelength of X-rays beams of light
lambda=0.824*10^-10  //m
//glancing angle of the incident light
theta1=(8+35/60)*(%pi)/180  //radians
n1=1
//to find theta3 i.e at
n3=3
//as we know that
//2*d*sin(theta)=n*lambda
//so for n1 and n3 we get in the same way and solving together we get 
theta3=asin(3*sin(theta1))
//so 
d=lambda/2/sin(theta1)
printf ("the galncing angle for thethird order diffraction is and interplanar spacing of the crystal is 2.761 A")