Question

When light propagates through a given homogeneous medium, the velocities of

• A. Primary wavefronts are lesser than those of secondary wavelets
• B. Primary wavefronts are greater than or equal to those of secondary wavelets.
• C. Primary wavefronts and wavelets are equal
• D. Primary wavefronts are larger than those of secondary wavelets

Answer 1

The Correct Option is B

Concept: 

According to Huygens' principle, every point on a primary wavefront acts as a source of secondary wavelets. The new position of the primary wavefront after a given time interval is given by the envelope (common tangent) of all these secondary wavelets.

Key Point:

• In a homogeneous medium, the speed of the primary wavefront and secondary wavelets are equal. This ensures that the new wavefront formed (by the secondary wavelets) moves forward smoothly and consistently.
• However, mathematically, the envelope (primary wavefront) can never move slower than the wavelets. It is either moving at the same speed (in homogeneous media) or faster (in specific geometrical conditions).

Hence, the correct option:

In a homogeneous medium, the velocities of primary wavefronts are greater than or equal to those of secondary wavelets.