Two slabs of equal thickness and cross-sectional area have thermal conductivities K1 and K2. If they are joined in series, the equivalent thermal conductivity of the combination is: MCQ with Answer and Explanation

Explanation:
T ∝ √m. If m' = 4m, T' = √4 × T = 2T. Period doubles when mass quadruples. Memory aid: 'Spring period: T ∝ √m; 4× mass ⇒ 2× period'. Oscillations proportionality frequently tested in competitive exams.

Explanation:
Niels Bohr proposed the Bohr model of the atom in 1913. He improved upon Rutherford's model by introducing quantum theory, suggesting that electrons travel in discrete, quantized orbits around the nucleus without radiating energy, and that energy is only emitted or absorbed when an electron jumps between these specific orbits.

Explanation:
Critical angle θ_c for total internal reflection: sinθ_c = n₂/n₁, where n₁ > n₂. Here n₁=1.5 (glass), n₂=1 (air), so sinθ_c = 1/1.5 = 2/3 ≈ 0.6667 ⇒ θ_c = sin⁻¹(0.6667) ≈ 41.8° ≈ 42°. Memory aid: 'sinθ_c = 1/n for glass to air'. This calculation tests understanding of total internal reflection conditions, crucial for optics in competitive exams. Always ensure light travels from denser to rarer medium for TIR to be possible; otherwise, no critical angle exists.