What Is Diffraction and When Does It Happen?

Diffraction is the spreading of a wave as it passes through an aperture or around an obstacle. It occurs for all waves but is most noticeable when the aperture width or obstacle size is similar to the wavelength.

The Reasoning

A wavefront passing through an aperture continues into the region beyond it. Near the edges, the wave changes direction and spreads into the geometrical shadow region rather than travelling only straight ahead.

The amount of diffraction depends mainly on the ratio between the wavelength $\lambda$ and the aperture width $b$:

For example, sound waves have relatively long wavelengths, so they diffract noticeably around doorways and buildings. Visible light has much shorter wavelengths, so everyday openings are usually much wider than its wavelength and the diffraction is difficult to observe.

Diffraction also occurs around obstacles. It becomes more significant when the obstacle's dimensions are comparable to the wavelength. This is why radio waves can bend around large structures more noticeably than visible light.

AHL extension: For single-slit diffraction, the approximate angle $\theta$ from the centre to the first minimum is

$$\theta \approx \frac{\lambda}{b}$$

for small angles. Increasing $\lambda$ or decreasing $b$ therefore increases the angular spreading.

A common misconception is that diffraction happens only when $b=\lambda$. It always occurs, but it becomes most noticeable when $b$ is comparable to or smaller than $\lambda$.

Exam Technique

This is C.3 Wave phenomena: qualitative diffraction is required at SL and HL, while quantitative single-slit diffraction is AHL. For an Explain question, identify wave spreading and link stronger diffraction to a larger $\lambda/b$ ratio. Do not confuse diffraction with refraction, which occurs when wave speed changes at a boundary.