CSIR NET Dec 2015 Q99: Electromagnetic methods
EM skin depth δ ≈ 503√(ρ/f) m.
δ ∝ √(ρ/f). Case 1: ρ/f = 100/1.2×10⁶. Case 2: ρ/f = 50/2.4×10⁶, which is 4 times smaller. So δ₂ = δ₁/√4 = δ₁/2, i.e. the skin depth is DECREASED by a factor of 2 (option 1). ⚠️ Note: The official key differs from the standard calculation/textbook answer. This solution follows the official NTA key; examiners may treat it differently. The official key gives option 2 (increased by a factor of 2).
1: Follows from δ ∝ √(ρ/f). 2: Official key. 3: ρ/f changes, so δ changes. 4: That would need ρ/f to fall 16 times.