According to Huygens' principle, what is the geometric shape of the wavefront originating from a point source located at a finite distance in an isotropic medium, and what shape does it take at an extremely large (infinite) distance?

Answer and explanation

Correct answer: C. Spherical wavefront at finite distance

  1. Step 1: A wavefront is defined as the locus of all adjacent points vibrating in the same phase.
  2. Step 2: In an isotropic medium (where speed of light is uniform in all directions), disturbances from a localized point source spread out symmetrically in three dimensions on spherical surfaces. Hence, the wavefront at a finite distance is spherical.
  3. Step 3: As the radius of curvature of the expanding sphere becomes exceptionally large (), any small localized portion of the spherical surface appears flat.
  4. Step 4: Therefore, at an infinite or very large distance from the point source, the wavefront approximates a plane wavefront.
Answer
Spherical wavefront at finite distance, becoming a plane wavefront at very large distance
Why not the others: 'Cylindrical wavefront' originates from an extended linear slit source, not a point source; 'Plane at finite distance' requires a collimated parallel beam; 'Ellipsoidal wavefront' only occurs in anisotropic crystals where light speed depends on direction.

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