What is diffraction?
Diffraction is the spreading out of waves as they pass through an opening (gap) or travel around the edge of an obstacle. After passing a gap, straight (plane) waves curve outward at the edges instead of continuing perfectly straight.
What affects diffraction?
The amount of spreading depends on two things:
- The size of the gap compared with the wavelength.
- The wavelength of the wave.
Diffraction is most pronounced when the gap width is about equal to the wavelength. A narrow gap gives strong spreading; a wide gap (much larger than λ) lets the waves pass through almost straight with only slight spreading at the edges.
Key formula / Key formula
During diffraction, frequency, wavelength and speed do NOT change — only the shape and direction of the wavefronts change.
Best spreading when gap width ≈ λ. Longer wavelength → more diffraction.
Worked example
In a ripple tank, water waves of wavelength 2 cm pass through a gap. Gap A is 2 cm wide and gap B is 8 cm wide. Which gap gives more diffraction?
Gap A: width (2 cm) ≈ wavelength (2 cm), so the waves spread out strongly.
Gap B: width (8 cm) is much larger than 2 cm, so the waves pass almost straight.
Gap A gives more diffraction.
Everyday examples
Sound diffracts around a doorway so you can hear someone before you see them. Radio waves (long wavelength) diffract easily around hills, but light (very short wavelength) diffracts only slightly, which is why shadows have fairly sharp edges.
Remember
- Diffraction changes the shape/direction, not the frequency, wavelength or speed.
- Narrower gap (near λ) → more diffraction.
- Longer wavelength → more diffraction.