The frequency of AC mains in India is 50 Hz. The time period is MCQ with Answer and Explanation

Explanation:
Geostationary satellites orbit in Earth's equatorial plane with period matching Earth's rotation period (24 hours), so they appear stationary relative to ground. Orbital radius is about 42,000 km from Earth's center. Memory tip: 'Geostationary: T=24 h, equatorial orbit, fixed position; used for communication/weather satellites'. This standard result is frequently tested in competitive exams. Always recall that geostationary is a special case of geosynchronous (same period) with zero inclination; competitive exams often compare low orbit (84 min) vs geostationary (24 h) periods.

Explanation:
Interference (constructive/destructive superposition) is wave phenomenon. Young's double-slit confirmed light's wave nature. Particle nature shown by photoelectric effect. Memory aid: 'Interference/diffraction ⇒ wave; photoelectric ⇒ particle'. Wave-particle duality frequently tested in competitive exams.

Explanation:
Volume constant: A' = A/2 when L' = 2L. R' = ρ(2L)/(A/2) = 4ρL/A = 4R. Memory tip: 'Stretch wire: R ∝ L² when volume fixed'. Resistance proportionality frequently tested in competitive exams.