LED works on the principle of MCQ with Answer and Explanation

Explanation:
Mirror formula: 1/f = 1/u + 1/v. Sign convention: f negative for concave mirror? Wait, standard Cartesian: f = -20 cm (concave), u = -30 cm (object real). Thus 1/(-20) = 1/(-30) + 1/v ⇒ -1/20 = -1/30 + 1/v ⇒ 1/v = -1/20 + 1/30 = (-3+2)/60 = -1/60 ⇒ v = -60 cm. Magnitude 60 cm, negative sign indicates real image in front of mirror. Many exams use magnitude-only options; here 60 cm is correct. Memory tip: 'Concave mirror: object beyond C (2f) ⇒ image between F and C, real, inverted'. This numerical tests mirror formula application with sign conventions, crucial for optics in competitive exams.

Explanation:
Specific gravity is a ratio of densities and is dimensionless. Strain (change in dimension/original dimension) is dimensionless. Plane and solid angles are also dimensionless quantities (ratio of lengths and areas, respectively). Wait, all are dimensionless. Let me re-evaluate. The question states 'does not have the same dimensions'. Since all four are completely dimensionless [M0L0T0], they actually DO have the same dimensions. Let's replace the question with a correct one. Let's use Planck's constant. Which of the following pairs have the same dimensions? A) Angular momentum and Planck's constant. B) Work and Power. C) Force and Torque. D) Momentum and Energy. Correct answer A. Angular momentum (mvr) is [ML^2T^-1]. Planck's constant (E/v) is [ML^2T^-1]. Both are identical.

Explanation:
Potentiometer uses null method: at balance, no current flows through cell, so measured voltage equals true EMF (no internal resistance drop). Memory aid: 'Null method ⇒ zero current ⇒ accurate EMF measurement'. Experimental physics concept frequently tested in competitive exams to verify understanding of precision measurement techniques.