Explanation:
Refractive index n = c/v, where c = 3×10⁸ m/s (speed in vacuum). Thus v = c/n = (3×10⁸)/(4/3) = 3×10⁸ × 3/4 = 9/4 × 10⁸ = 2.25×10⁸ m/s. This direct formula application is fundamental in optics. Memory aid: 'Higher n ⇒ slower light in medium'. Competitive exams frequently test this calculation with common refractive indices (water 4/3, glass ~1.5). Always use c = 3×10⁸ m/s unless specified otherwise, and ensure unit consistency (m/s here).
Explanation:
Least count of screw gauge = Pitch / Number of circular scale divisions. Given pitch = 1 mm, divisions = 100, so LC = 1 mm / 100 = 0.01 mm. This represents the smallest measurement the instrument can accurately detect. Screw gauges measure small dimensions like wire diameter with high precision. Memory tip: Least count formula is universal for vernier and screw instruments: value per division on main scale divided by total circular divisions. This concept is frequently tested in practical physics sections of competitive exams.
Assertion (A): Light year and astronomical unit both represent dimensions of length. Reason (R): Light year is the time taken by light to travel from the Sun to the Earth.
A.Both A and R are true and R is the correct explanation of A.
B.A is true but R is false.
C.Both A and R are true but R is NOT the correct explanation of A.
Explanation:
Assertion (A) is true because both light year and astronomical unit (AU) are units used to measure massive astronomical distances (lengths). Reason (R) is false because a light year is the distance light travels in a vacuum in one Earth year, not the time taken to travel from the Sun to Earth (which is about 8.3 minutes).
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