Huygens principle

Huygens’ Principle says:

👉 Every point on a wavefront acts like a tiny source of new little waves (called wavelets).

👉 These small wavelets spread out in all directions.

👉 The next wavefront is formed by drawing a surface that just touches all the wavelets.

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How this explains diffraction

Diffraction = waves spreading out when they pass through a gap or around an obstacle.

Using Huygens’ Principle:

✔ 1.

Wavefront reaches a gap

Imagine a straight wave approaching a narrow opening.

When the wavefront reaches the gap:

• Only the points inside the gap can act as sources of wavelets.

• The points blocked by the barrier cannot produce waves.

✔ 2.

Points inside the gap create new circular wavelets

These wavelets spread out in all directions from the gap.

✔ 3.

These wavelets overlap and form a curved wavefront

Because the gap is small, the wavelets spread widely → the wave seems to “bend.”

✔ 4.

Result = Diffraction

The wave emerges from the opening as if the gap itself were a new wave source.

This makes the wave spread out, not continue straight.

• Small gap ≈ more spreading because fewer wavelets → each spreads more.

• Large gap ≈ less spreading because many wavelets → behave almost like a straight wavefront.

“Huygens’ principle states that every point on a wavefront acts as a source of secondary wavelets. When a wave passes through a small gap, only the points at the gap produce wavelets, which spread out in all directions. These wavelets form a new curved wavefront, causing the wave to diffract.”