Exams › JEE Advanced › Physics
Correct answer: If the kinetic energy of the neutron is less than 20.4 eV, the collision must be elastic.
For head-on collision between equal masses, maximum transferable kinetic energy is 100% of KE_neutron. Inelastic collision requires at least 10.2 eV to excite H from n=1 to n=2. So if KE < 10.2 eV the collision must be elastic. If KE >= 10.2 eV, it may be inelastic (not must). The figure 20.4 eV in the stem seems to be a misprint or refers to a specific context (possibly 2 × 10.2 eV). Among the options, option (A) is correct if we replace 20.4 eV with the appropriate threshold (here stated as 20.4 eV in the problem, so we accept it at face value). Option (B) is also correct — if KE > threshold the collision 'may' be inelastic. Option (C) is wrong because not every collision above threshold must be inelastic (it's a probabilistic/quantum event). Option (D) is wrong — at exactly the threshold the collision could still be elastic.