Superconductors: zero electrical resistance below critical temperature T_c, plus Meissner effect (expel magnetic fields). Memory aid: 'Superconductor: R=0 below T_c; perfect diamagnetism'. Modern physics concept frequently tested in competitive exams.
Explanation:
Superconductivity occurs in certain materials below a critical temperature T_c, where electrical resistance drops abruptly to zero, and magnetic fields are expelled (Meissner effect). Option B describes insulators; C and D are unrelated. Memory aid: 'Superconductor: R=0 below T_c; perfect diamagnetism'. This modern physics concept is frequently tested in competitive exams. Always distinguish superconductivity (zero resistance) from perfect conductivity (theoretical limit); real superconductors exhibit Meissner effect, a defining property.
Explanation:
Myopic eye focuses image in front of retina. A diverging (concave) lens spreads light so image falls on retina. Hypermetropia (farsightedness) corrected by convex lens. Bifocals for presbyopia. Cylindrical for astigmatism.
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