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.

Answer 2

When light passes through a homogeneous medium, according to Huygens' principle, every point on a wavefront acts as a source of secondary wavelets. These secondary wavelets spread in all directions, and the primary wavefront is formed as the surface tangent to these secondary wavelets at any given instant.

The velocity of the primary wavefront is always greater than or equal to the velocity of the secondary wavelets, because the secondary wavelets propagate from the points on the primary wavefront, and their velocity is dependent on the medium's properties. The velocity of secondary wavelets is generally considered slower or equal to the primary wavefronts.

Thus, the correct statement is that primary wavefronts are greater than or equal to those of secondary wavelets.