Define the term wave front. State Huygen's principles.

Explanation

### Step-by-Step Solution:

**Step 1: Define Wavefront**
- A wavefront is defined as the locus of all points in a medium that are reached by a wave at the same time. In simpler terms, it represents the surface over which an oscillation or wave has the same phase.

**Step 2: Visualize Wavefront**
- To visualize this, consider a point source of light (denoted as point O) that emits waves in all directions. The wavefront can be represented as a surface that connects all points in the medium that the wave has reached simultaneously. For a point source, this wavefront takes the shape of a sphere expanding outward.

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