8.2 Diffraction

Subject: Cambridge International AS Level Physics (9702) Topic: 8 – Superposition

---

📖 Main Content

---

1. What is Diffraction?

Diffraction is the spreading out of waves when they pass through a narrow opening (called a slit or aperture — simply a gap in a barrier) or when they travel around the edge of an obstacle (a barrier blocking their path).

Think of it this way: imagine waves on the surface of the sea heading towards a harbour entrance. Instead of just passing straight through the gap and continuing in a straight line, the waves bend and fan outward into the harbour on the other side. That spreading is diffraction.

---

Diffraction Through a Gap

When straight waves (called plane waves — waves whose fronts are flat, parallel lines moving in one direction) hit a barrier with a gap:

• The part of the wave that lines up with the gap passes through.
• Instead of continuing in a perfectly straight line, the wave spreads outward on the other side of the gap.
• The wavefronts on the other side become curved — often semi-circular — rather than staying flat.

Imagine it: You are standing behind a wall that has a small doorway. You can still hear someone talking on the other side even if you are off to the side — because sound waves spread out (diffract) through the doorway opening.

---

Diffraction Around an Edge

Diffraction does not only happen through gaps. It also happens when waves travel past the edge of a barrier:

• Waves approaching the edge of a single barrier will curve around that edge into the region behind the barrier.
• This means waves can reach areas that are not directly in front of the wave's original direction of travel.

Everyday example: Radio waves travel around the edges of hills and buildings, which is why you can sometimes receive a radio signal even when the transmitter is not in your direct line of sight.