Photoelectric threshold frequency ν₀ = φ/h. If work function doubles, ν₀: MCQ with Answer and Explanation

Explanation:
LED: electroluminescence - electron-hole recombination in semiconductor emits photons. Incandescence: thermal radiation (bulbs); Fluorescence: UV excitation. Memory tip: 'LED = semiconductor light emission; no filament, efficient'. Modern device principle frequently tested in competitive exams linking physics to technology.

Explanation:
Ionosphere contains ions and free electrons, reflects radio waves back to Earth, enabling long-distance radio communication. Used in skywave propagation. Troposphere weather layer.

Explanation:
A Venturimeter consists of a converging pipe, a narrow throat, and a diverging pipe. As fluid passes through the constricted throat, its velocity increases. According to Bernoulli's principle, this increase in kinetic energy must be accompanied by a strictly corresponding decrease in fluid pressure. By measuring this pressure difference, the flow rate can be precisely calculated.