PhysicsChapter 9: Waves
Diffraction
Explanation level
Meaning of diffraction
According to geometrical propagation, a wave travelling in straight lines should leave a sharp shadow behind an obstacle. Real waves bend around edges and spread into this shadow region. The same spreading occurs when a wave passes through a slit.
Condition for observable diffraction
Let a represent the width of an opening or the relevant size of an obstacle, and let λ be the wavelength. Diffraction becomes clear when these two lengths are comparable.
- a
- Width of slit or size of obstacle (m)
- λ
- Wavelength (m)
| Relative size | Wave behaviour |
|---|---|
| a is comparable to λ | Strong, easily observed diffraction |
| a is much larger than λ | Small bending mainly near the edges |
| a is smaller than or close to λ | Very broad spreading after the opening |
Water-wave diffraction in a ripple tank
Plane water waves are generated in a ripple tank. Two barriers are placed in line so that the gap between them forms a slit. The slit width can be compared with the wavelength of the incoming waves.
Narrow slit
When the separation between the barriers is approximately equal to the wavelength, the plane wavefronts spread strongly after passing through the gap. The outgoing wavefronts become almost semicircular.
Wide slit
When the opening is much wider than the wavelength, the central part of each wavefront continues almost straight. Only the portions near the edges bend noticeably, so the overall diffraction is small.
| Ripple-tank opening | Outgoing wavefront |
|---|---|
| Narrow and comparable to λ | Strongly curved or almost semicircular |
| Much wider than λ | Mostly straight with slight edge curvature |
Diffraction of light
Light also diffracts, but its wavelength is very small. Clear diffraction therefore requires a very narrow slit or a sharp edge whose size is comparable to the wavelength of the monochromatic light used.
A beam of monochromatic light passes through a single slit and falls on a screen or photographic film. Instead of forming only a narrow geometrical image of the slit, it spreads into a pattern of bright and dark bands.
Single-slit diffraction pattern
- The central bright band has the greatest intensity.
- The central band is wider than the geometrical slit image and wider than the surrounding bands.
- Weaker bright bands appear on both sides of the centre.
- Dark bands separate neighbouring bright bands.
Why bright and dark bands form
Every part of the slit opening contributes a spreading wave. At the screen, these contributions superpose. In some directions they reinforce one another and form bright bands; in others they cancel and form dark bands. The observed diffraction pattern therefore contains interference effects.
What the single-slit pattern shows
Observation: Light spreads beyond the geometrical slit image and forms alternating bright and dark bands.
Experiment: Monochromatic light passes through a narrow slit and reaches a distant screen.
Result: A broad, intense central band and weaker surrounding bands appear.
Conclusion: Light diffracts and its spreading components interfere, demonstrating wave behaviour.
Diffraction and wavelength
For the same opening, a wave with a longer wavelength has a larger wavelength-to-aperture ratio and spreads more. This is why sound bends around everyday obstacles easily, while visible-light diffraction usually needs very small openings.
| Wave | Typical wavelength | Everyday diffraction |
|---|---|---|
| Sound | Relatively long | Easily heard around doors and corners |
| Water waves | Visible and adjustable | Easy to demonstrate in a ripple tank |
| Visible light | Very short | Requires a narrow slit or fine obstacle |
Conceptual example
Changing the ripple-tank slit
Situation: Plane water waves of fixed wavelength pass first through a narrow gap and then through a much wider gap.
Explanation: The narrow gap produces almost semicircular wavefronts because its width is comparable to the wavelength. The wide gap produces nearly straight wavefronts with bending only near the edges.
Common mistakes
Check your understanding
Q. What is diffraction?
A. It is the bending and spreading of waves around obstacles or through openings into the geometrical-shadow region.
Q. When is diffraction most noticeable?
A. When the obstacle or aperture size is comparable to the wavelength.
Q. What happens to water waves after a narrow slit?
A. Plane wavefronts spread and become almost semicircular.
Q. What are the main features of a single-slit light pattern?
A. It has a broad, intense central bright band with weaker bright bands and dark bands on both sides.
Quick revision
- Diffraction is the bending and spreading of waves around obstacles or through openings.
- Diffracted waves enter regions that geometrical straight-line propagation would label as shadow.
- Diffraction is strongest when the obstacle or opening size is comparable to the wavelength.
- A narrow slit in a ripple tank changes plane water waves into strongly curved, almost semicircular wavefronts.
- A wide slit compared with the wavelength produces only slight bending near its edges.
- Monochromatic light diffracts when it passes through a sufficiently narrow single slit or near a sharp edge.
- A single-slit light pattern has a bright, broad central band and weaker surrounding bands.
- The bright and dark bands arise from interference among waves spreading from different parts of the slit.
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Exam preparation
Exam Focus
Important points
- Definition of diffraction.
- Strong-diffraction condition a≈λ.
- Narrow versus wide ripple-tank slit.
- Light diffraction through a narrow single slit.
- Central maximum is widest and most intense.
Common mistakes
- Restricting diffraction to slits only.
- Claiming wide openings give the greatest spreading.
- Confusing diffraction with refraction.
- Calling the central bright band the narrowest.
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Diffraction
Sticky Notes · Quick Revision
Key Point 1
- Diffraction is the bending and spreading of waves around obstacles or through openings.
Key Point 2
- Diffraction is strongest when the obstacle or opening size is comparable to the wavelength.
Key Point 3
- A narrow slit in a ripple tank changes plane water waves into strongly curved, almost semicircular wavefronts.
Key Point 4
- A single-slit light pattern has a bright, broad central band and weaker surrounding bands.
Ready to test yourself?
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- 8 concept questions
- Easy → Medium → Hard
- Explanation after each answer
Exam Practice
Important Question
Q1. Define diffraction and state the condition for observing it clearly.
Important Question
Q2. Describe diffraction of water waves in a ripple tank.
Important Question
Q3. Describe the diffraction pattern produced by a single slit.
Reasoning
Q4. Why is sound diffraction common while light diffraction needs a very narrow slit?
Comparison
Q5. Differentiate between diffraction and refraction.
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