Physics and Mathematics
Difference Between Interference and Diffraction of Light
1. Concept Overview
Both interference and diffraction are wave phenomena that occur due to superposition of light waves.
However, their origin and interpretation are different:
- Interference occurs due to superposition of waves from two or more coherent sources.
- Diffraction occurs due to superposition of secondary wavelets from different parts of the same wavefront (usually after passing through a single slit or aperture).
2. Clear Explanation
| Feature | Interference | Diffraction |
|---|---|---|
| Cause | Superposition of waves from two coherent sources | Bending and spreading of light from one aperture or obstacle |
| Fringe Width | Almost constant | Not constant — central maximum is widest |
| Intensity Distribution | Bright and dark fringes of almost equal intensity | Intensity decreases for successive maxima |
| Source Requirement | Requires two coherent sources (e.g., Young’s experiment) | Usually requires only one slit |
| Effect of Slit Width | Does not affect fringe width much | Strongly affected by slit width |
| Condition for Maximum/Minimum | Path difference condition | Diffraction condition like [a sinθ = mλ] |
| Mathematical Treatment | Simple interference formulas | Involves Huygens–Fresnel principle and integration |
| Occurrence | More pronounced when sources are close and coherent | More pronounced when aperture size ~ wavelength |
| Central Bright Spot | Nearly same width as other fringes | Widest and brightest in diffraction |
| Practical Use | Used in optical testing, thin films | Used in resolving power of optical instruments |
3. Dimensions & Units
Not applicable — comparison topic.
4. Key Features
- Interference: Redistribution of light energy from two sources
- Diffraction: Modification due to spreading of light from a single wavefront
- Diffraction effect becomes negligible if aperture >> wavelength
- Interference requires coherent sources; diffraction does not
5. Important Points to Remember (Quick Table)
| Property | Interference | Diffraction |
|---|---|---|
| Central Maximum Width | Nearly same as others | Largest |
| Intensity Pattern | Uniform variation | Non-uniform |
| Fringes Visibility | Better contrast | Lesser contrast |
| Energy Consideration | Redistribution with conservation | Redistribution with conservation |
6. Conceptual Questions with Solutions
1. Why is central fringe more intense in diffraction than interference?
In diffraction, main contribution to central region comes from **entire aperture** → more constructive interference → high intensity.
2. Why must sources be coherent for interference but not for diffraction?
Interference requires stable phase difference from **two independent sources**. Diffraction uses **one aperture**, so coherence is inherent.
3. Why fringe width remains nearly constant in interference?
Fringe width [β = Dλ/d] depends only on **source separation**, which remains constant.
4. Does diffraction occur in absence of interference?
No. Diffraction **is itself** an interference among secondary wavelets from different parts of same wavefront.
5. Why diffraction cannot be ignored in optical instruments?
It causes **blurring** and limits ability to resolve two close objects.
6. Which pattern will have highest contrast: interference or diffraction?
Interference — because bright fringes have higher intensity relative to dark fringes.
7. Why do we observe diffraction more distinctly from sound?
Sound has **larger wavelength** compared to everyday objects → noticeable bending.
8. When interference pattern becomes similar to diffraction?
When the number of slits increases (multiple-slit interference → diffraction envelope).
9. Do both interference & diffraction conserve energy?
Yes. Light energy is **redistributed** among maxima and minima.
10. If slit width decreases, which effect dominates: interference or diffraction?
Diffraction becomes dominant because **W ∝ 1/a**.
7. FAQ / Common Misconceptions
1. Interference and diffraction are totally different phenomena. True?
False. **Diffraction is due to interference** of secondary wavelets.
2. Diffraction happens only with narrow slits. True?
False. It happens always — narrow slits make it more noticeable.
3. Interference does not occur with a single slit. True?
False. Diffraction in a single slit **is** interference internally.
4. Intensity always remains uniform in diffraction. True?
False. It **drops** for higher-order maxima.
5. For interference, dark fringe means no light arrives. True?
False. Light does arrive but gets **cancelled** due to destructive interference.
6. If interference disappears, diffraction also disappears. True?
False. Diffraction persists even without two sources.
7. Diffraction pattern can occur even without coherent light. True?
Yes — coherence is naturally produced within a single slit.
8. Large aperture gives strong diffraction. True?
False. Smaller aperture gives strong diffraction.
9. Interference pattern cannot show diffraction effects. True?
False. In YDSE, each slit also produces diffraction → overall pattern modulated by diffraction envelope.
10. Diffraction pattern has infinite maxima. True?
True, but **intensity decreases rapidly** for higher orders.
8. Practice Questions (With Step-by-Step Solutions)
Q1. Write one key point of difference between interference and diffraction based on intensity distribution.
Solution:
Interference → almost equal bright fringes
Diffraction → central maximum brightest, intensity decreases for next maxima.
Q2. A monochromatic source is used in Young’s experiment. Which phenomenon is observed?
Solution:
Interference, because there are two coherent sources (two slits).
Q3. Which pattern will become more dominant if slit width decreases?
Solution:
Diffraction (since [Δθ ∝ 1/a]).
Q4. Why cannot diffraction be ignored in telescopes?
Solution:
Diffraction limits resolution, causing blurring of fine details.
Q5. Give one example of diffraction from daily life.
Solution:
Hearing sound around a corner.
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