15. A team of researchers would like to design a double-slit apparatus such that the pattern contains precisely seven fringes in the central diffraction peak. The slit width a and separation d are related by d = na. Which one of the following value n is impossible? (A) n = 3.1 (B) n = 3.9 (C) n = 4.2 (D) n = 4.0

10. The number of collisions a molecule makes per second is called collision frequency. At one atmosphere and 0 °C, estimate the collision frequency of air molecules. [The diameter of an air molecule under one atmosphere and 0 °C is approximately 3×10⁻¹⁰ (m), the average molecular speed is approximately 460 (m/s), and the molecular number density is approximately 2.7×10²⁵ molecules/m³]. (A) 3.5×10⁵ (collisions/s) (B) 3.5×10⁷ (collisions/s) (C) 3.5×10⁹ (collisions/s) (D) 3.5×10¹¹ (collisions/s)

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