Lesson summary
8.2.1 Diffraction — AS PHYSICS 9702 notes: Describe diffraction of waves at gaps and edges and how its extent depends on gap width and wavelength.Definition — the spreading out of waves when they pass an obstruction, typically a narrow slit or aperture.Extent — it depends on the …
- Most prominent
- Only amplitude changes
- Changing the effect
- UV and gate posts
- Radio wave
- Ripple tank
Section 1 of 8What is diffraction
Definition
- Definition the spreading out of waves when they pass an obstruction, typically a narrow slit or aperture.
Extent
- Extent it depends on the width of the gap compared with the wavelength.
Most prominent
- Most prominent when the slit width is approximately equal to the wavelength.
Edges
- Edges diffraction also occurs when waves curve around the edge of a barrier.
Any wave
- Any wave sound, light and water waves can all be diffracted.
Section 2 of 8Effect on the wave
Only amplitude changes
- Only amplitude changes wavelength, frequency and speed stay the same.
Reason
- Reason some energy is dissipated when the wave passes through the gap.
Drawing
- Drawing keep the wavelength constant when drawing diffracted wavefronts.
Section 3 of 8Gap size and wavelength
| Gap compared with wavelength | Effect |
|---|---|
| Much smaller | The wave spreads out significantly after passing through. |
| About equal | Diffraction is most pronounced. |
| Much bigger | The wave passes through with little diffraction. |
Section 4 of 8Ripple tank demonstration
Use
- Use a ripple tank demonstrates diffraction of water waves through a gap and around a barrier.
Changing the effect
- Changing the effect change the wavelength compared with the gap size.
Wide gap
- Wide gap when the gap is bigger than the wavelength, the waves spread out less.
Example
- Example water waves moving through a gap into a harbour.
Section 5 of 8Choosing waves that diffract
Method
- Method compare the wavelength with the size of the gap.
X-rays
- X-rays wavelengths of 10−8 m to 4 × 10−13 m are close to the spacing between atoms in a crystal, so they diffract there.
UV and gate posts
- UV wavelengths are far too small to diffract at a gate post.
Sound
- Sound sound wavelengths (about 1.7 × 10−2 m to 17 m) are far larger than the slits of a diffraction grating.
Section 6 of 8More from the specification
More from the specification
Diffraction at Slits and Obstacles
- Diffraction is the spreading of wavefronts as they pass through an aperture or around the edge of an obstacle;
- diffraction effects become most pronounced when the aperture width is approximately equal to the wavelength.
Diffraction Demonstrations in Ripple Tanks
- Ripple tanks provide clear visual demonstrations of water wave diffraction,
- where plane straight wavefronts passing through a narrow barrier slit spread out into circular wavefronts.
Radio Wave Propagation and Obstacle Diffraction
- Long-wavelength radio waves diffract around large physical structures such as hills and buildings,
- allowing radio broadcast signals to reach ground receivers located beyond direct line of sight.
X-Ray Diffraction in Crystalline Solids
- X-rays possess wavelengths (~10-10 m) comparable to the atomic plane spacing in a crystalline solid, undergoing constructive and destructive interference upon diffraction to produce characteristic crystal structure patterns.
Radio wave
- Longest-wavelength electromagnetic wave
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Diffraction | Spreading out of waves when they pass through a gap or around an edge. |
| Aperture | Gap or slit through which a wave passes. |
| Wavefront | Line joining points of a wave in the same phase. |
| Ripple tank | Shallow water tank used to show wave properties. |
Section 8 of 8Exam technique
1
Changing the wavelength of the wave when drawing diffraction — only the amplitude changes.
2
Saying diffraction needs a gap much smaller than the wavelength — it is largest when similar.
3
Thinking only water waves diffract — all waves do.
4
Thinking a wide gap produces strong spreading — wide gaps produce little diffraction.