(b) The first order reflection of a beam of Xrays from 100 planes of NaCl occurs at an angle of 6°30'. Calculate the wavelength of the X-ray. What would be the angle of reflection if X-rays of 1:54A were used. 4 What is radius ratio ? How does radius ratio help in determining the structure of ionic solids and co-ordination number of ions ? Explain. 3 (d) An element crystallizes in a body centered cubic structure with a cell edge of 288 pm. The density of the element is 7-2gcm. How many atoms are prescnt in 208g of the element?