A wire of resistance 9 Ω is bent to form an equilateral triangle. The resistance across any two vertices is MCQ with Answer and Explanation

Explanation:
Total internal reflection (TIR) requires: (1) light travels from denser to rarer medium (n₁ > n₂), and (2) angle of incidence exceeds critical angle θ_c = sin⁻¹(n₂/n₁). Option A describes refraction toward normal; B describes partial reflection/refraction; D is incorrect. Memory aid: 'TIR: denser→rarer AND i > θ_c'. This condition-based question tests optics fundamentals, frequently examined in competitive exams. Always verify both conditions for TIR; common applications include optical fibers and prisms in binoculars.

Explanation:
A concave mirror forms real and inverted images in most positions. The only exception is when the object is placed very close to the mirror, specifically between the pole (P) and the principal focus (F). In this position, the reflected rays diverge, forming a virtual, erect, and magnified image behind the mirror. This is why concave mirrors are used as shaving or makeup mirrors.

Explanation:
Raman effect: inelastic scattering of light, 1928, Nobel 1930.