**Huygens' principle** states that every point on a wavefront acts as a:
- Source of plane waves only
- Secondary source emitting wavelets in all directions
- Absorber of waves
- Source of longitudinal waves only
Show answer and solution
Correct answer: B. Secondary source emitting wavelets in all directions
Each point on a wavefront becomes a secondary source of spherical wavelets.
The new wavefront at a later time is the **envelope** of these wavelets in the forward direction (backward wavelets are taken as ineffective).