At what depth below the surface of the Earth does the acceleration due to gravity become zero? MCQ with Answer and Explanation

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At what depth below the surface of the Earth does the acceleration due to gravity become zero?

A. It never becomes zero

B. At the center of the Earth

C. At a depth equal to one-fourth the radius of the Earth

D. At a depth equal to half the radius of the Earth

Answer: Option B

Solution (By JKSSB Mock Tests)

The acceleration due to gravity at a depth 'd' is given by g' = g(1 - d/R), where R is the radius of the Earth. If we go to the center of the Earth, the depth 'd' becomes exactly equal to R. Substituting d = R into the formula gives g' = g(1 - R/R) = g(1 - 1) = 0. So, gravity is zero at the core.

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The phenomenon of diffraction limits the resolving power of optical instruments because:

A. Lenses absorb light

B. Atmospheric turbulence distorts light

C. Mirrors reflect imperfectly

D. Light bends around edges, causing image blurring

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Correct Answer: Option D

Explanation:
Diffraction causes light waves to spread when passing through apertures or around obstacles, creating Airy patterns instead of point images. This fundamental wave effect limits the minimum resolvable detail (Rayleigh criterion: θ_min ≈ 1.22λ/D). Options B, C, D are practical limitations but not the fundamental diffraction limit. Memory tip: 'Diffraction limit: smaller λ or larger aperture ⇒ better resolution'. This wave optics concept is frequently tested in competitive exams. Always distinguish fundamental limits (diffraction) from technical limitations (aberrations, turbulence) in instrument performance.

This question belongs to: Science Physics