A one-dimensional linear chain of atoms contains two types of atoms of masses m, and m, (where m, > m,), arranged altemately. The distance between successive atoms is the same. Assume that the harmonic approximation is valid. At the first Brillouin zone boundary, which of the following statements is correct? (a) The atoms of mass m2, are at rest in the optical mode, while they vibrate in the acoustical mode. (b The atoms of mass m, are at rest in the optical mode, while they vibrate in the acoustical mode. office Branch office Anand Institute of Mathematio 28-B/6, Jia Sarai, Near lIT Hauz Khas, New Delhi-16 H.No. 23, G.F, Jia Sarai, IIT, Hauz Khas, New Delhi-16 e: 011-26865455/+91-9871145498 Website: www.physicsbyfiziks.com Email: email@example.com 16 fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES (C) Both types of atoms vibrate with equal amplitudes in the optical as well as in the acoustical modes. (d) Both types of atoms vibrate, but with unequal, non-zero amplitudes in the optical as well as in the acoustical modes.