FORCE

🌈 Diffraction of Light 🔬

✨ Definition • Types • Formula • Examples • Applications ✨

💡 Introduction: Light is an electromagnetic wave that normally travels in straight lines through a uniform medium. However, when light passes through a narrow opening or around an obstacle, it spreads into the surrounding region. This fascinating phenomenon is called diffraction of light.

🔵 1. What Is Diffraction of Light?

Diffraction of light is the bending and spreading of light waves when they pass through a narrow slit or move around the edge of an obstacle. It is an important property of waves and demonstrates the wave nature of light.

📘 Definition: Diffraction is the spreading of light waves into the region beyond an aperture or around an obstacle, particularly when its size is comparable to the wavelength of light.

🌟 The wavelength of visible light is approximately 400–700 nanometres. Diffraction is especially noticeable when the opening or obstacle has dimensions comparable to this wavelength.

🟣 2. Why Does Diffraction Occur?

According to the wave theory of light, every point on a wavefront can be treated as a source of secondary wavelets. When light passes through a narrow slit, these wavelets spread out and overlap. Their interference produces bright and dark regions called a diffraction pattern.

✨ Remember: A narrower slit generally produces greater angular spreading. Diffraction is much less noticeable when the opening is very large compared with the wavelength.

🟢 3. Types of Diffraction

🌿 A. Fresnel Diffraction

Fresnel diffraction is also called near-field diffraction. It occurs when the source and observation screen are at finite distances from the diffracting object, and the curvature of the wavefront must be taken into account.

🌊 B. Fraunhofer Diffraction

Fraunhofer diffraction is also called far-field diffraction. The incident and diffracted wavefronts are treated as approximately plane. This type is commonly studied using lenses in optical experiments.

🌿 Fresnel	🌊 Fraunhofer
Near-field diffraction	Far-field diffraction
Wavefront curvature is important	Wavefronts are approximately plane
Pattern varies with observation distance	Often studied in a lens's focal plane

🔴 4. Single-Slit Diffraction

When monochromatic light passes through a narrow slit, a pattern of bright and dark fringes appears on a screen. The central bright maximum is usually the widest and brightest part of the pattern. Weaker secondary maxima appear on both sides.

a sin θ = mλ

Condition for dark fringes in single-slit diffraction

📌 Meaning of symbols:

• a = width of the slit
• θ = diffraction angle
• λ = wavelength of light
• m = ±1, ±2, ±3, ... for dark fringes

The formula shows that the diffraction angle increases as the slit width decreases, for a given wavelength.

🟠 5. Factors Affecting Diffraction

🔴 1. Wavelength: Longer wavelengths generally produce greater angular spreading for the same slit width.

🟡 2. Slit Width: A narrower slit produces a wider angular diffraction pattern.

🟢 3. Screen Distance: In a far-field setup, increasing the screen distance increases the linear size of the pattern.

🔵 4. Shape of the Opening: The shape and size of an aperture influence the resulting pattern.

🌈 6. Real-Life Examples of Diffraction

💿 Example 1: Colours on a CD or DVD

The closely spaced tracks on a CD or DVD act as a diffraction grating. They separate different wavelengths of white light, producing beautiful rainbow colours.

🔬 Example 2: Optical Instruments

Diffraction limits the resolution of microscopes, telescopes and cameras. It determines how closely spaced details can be distinguished.

💡 Example 3: Light Through a Narrow Slit

When a narrow slit is illuminated with monochromatic light, alternating bright and dark fringes appear on a screen.

🌑 Example 4: Light Near an Obstacle

Light can spread into the geometrical shadow region of a small obstacle. This effect becomes significant when the obstacle's dimensions are comparable to the wavelength.

🟡 7. Diffraction vs Interference

🌈 Diffraction	✨ Interference
Waves spread at an aperture or obstacle.	Two or more waves overlap.
Often involves different parts of the same wavefront.	Often demonstrated using coherent wave sources.
Single-slit patterns have a broad central maximum.	Ideal double-slit fringes are equally spaced.

💡 Important fact: Diffraction and interference are closely related. A diffraction pattern is itself formed by the interference of different parts of a wavefront.

🟣 8. Applications of Diffraction

• 🔬 Determining the wavelength of light.
• 💎 Studying crystal structures using X-ray diffraction.
• 🔭 Understanding the resolution of optical instruments.
• 🌈 Separating wavelengths using diffraction gratings.
• 🧪 Analysing the properties and structures of materials.
• 📡 Designing spectrometers and optical devices.

🏆 9. Conclusion

Diffraction of light is a fascinating wave phenomenon that demonstrates the wave nature of light. It occurs when light spreads after passing through a narrow opening or around an obstacle.

🌟 Understanding diffraction helps us explain optical patterns, the resolving power of scientific instruments, spectroscopy and many modern technologies.

📚 Quick Revision Notes

• 🌈 Diffraction is the spreading of waves at an aperture or obstacle.
• 🔬 It provides evidence for the wave nature of light.
• 🌿 Fresnel diffraction is associated with near-field conditions.
• 🌊 Fraunhofer diffraction is associated with far-field conditions.
• 📐 Single-slit minima satisfy a sin θ = mλ.
• 💡 Narrower slits generally produce greater angular spreading.

🏷️ Blog Labels:
Diffraction of Light | Wave Optics | Physics Notes | Single-Slit Diffraction | Fresnel Diffraction | Fraunhofer Diffraction | Science Education

🌈 Practice Questions: Diffraction of Light

📚 Physics | Wave Optics | 20 Questions

Test your knowledge and check your score!

📝 Instructions:

• There are 20 questions, each worth 1 mark.
• Select one answer for each multiple-choice question.
• Questions 16–20 test numerical and conceptual understanding.
• Click Submit Answers to see your score.
• Use the answer key after attempting the quiz.

🔵 Section A: Multiple-Choice Questions

1. What is diffraction of light?

Reflection of light from a mirror Spreading of light waves around an obstacle or through an aperture Splitting of white light only by a prism Absorption of light by a surface

2. Diffraction is most noticeable when the aperture size is:

Much larger than the wavelength Comparable to the wavelength Always exactly one metre Independent of wavelength

3. Which phenomenon provides evidence for the wave nature of light?

Diffraction Photoelectric emission alone Rectilinear propagation alone Absorption alone

4. Which type of diffraction is associated with near-field conditions?

Fraunhofer diffraction Polarisation Fresnel diffraction Refraction

5. Which type of diffraction is commonly studied in the far field?

Fresnel diffraction Fraunhofer diffraction Total internal reflection Dispersion only

6. What is the condition for minima in single-slit diffraction?

a sin θ = mλ, where m = ±1, ±2, ... a cos θ = λ for every minimum a sin θ = 0 for every minimum a = m + λ

7. What happens when the width of a slit decreases?

Angular spreading generally increases Angular spreading always becomes zero The wavelength becomes zero Diffraction becomes impossible

8. In a single-slit diffraction pattern, the central maximum is generally:

The narrowest dark band The widest bright maximum Always absent A point with zero intensity

9. The colourful pattern seen on a CD under white light is mainly due to:

Polarisation only Total internal reflection only Diffraction and interference from closely spaced tracks Radioactive emission

10. Which instrument is fundamentally limited in resolution by diffraction?

Thermometer Barometer Simple spring balance Optical microscope

🟣 Section B: True or False

11. Diffraction can cause light to spread into a geometrical shadow region.

True False

12. For the same slit width, shorter wavelengths generally diffract through larger angles than longer wavelengths.

True False

13. Diffraction and interference are related wave phenomena.

True False

14. In single-slit diffraction, the central maximum is generally narrower than each adjacent secondary maximum.

True False

15. Fraunhofer diffraction is commonly treated as a far-field phenomenon.

True False

🟢 Section C: Numerical and Conceptual Questions

16. Light of wavelength 600 nm passes through a slit 0.20 mm wide. For the first minimum, approximately what is sin θ?

0.003 0.03 0.3 3

17. A slit has width 0.10 mm and is illuminated with light of wavelength 500 nm. For the first minimum, sin θ is approximately:

0.0005 0.005 0.05 0.5

18. In a single-slit experiment, the slit width is halved while wavelength stays constant. The angular width of the central maximum approximately:

Halves Doubles Remains unchanged Becomes zero

19. For a slit width of 0.25 mm and wavelength 500 nm, the first-minimum angle is approximately:

0.002 rad 0.02 rad 0.2 rad 2 rad

20. Which change would increase the angular width of the central diffraction maximum?

Increase slit width and shorten wavelength Increase slit width while keeping wavelength constant Decrease wavelength and increase slit width Decrease slit width while keeping wavelength constant

📖 Click here to reveal the answer key and explanations

1. B — Diffraction is the spreading of waves at an aperture or obstacle.

2. B — Diffraction is pronounced when aperture size is comparable to wavelength.

3. A — Diffraction is evidence of the wave nature of light.

4. C — Fresnel diffraction describes near-field conditions.

5. B — Fraunhofer diffraction describes far-field conditions.

6. A — Single-slit minima satisfy a sin θ = mλ for non-zero integers m.

7. A — A narrower slit increases angular spreading.

8. B — The central bright maximum is generally twice the width of a secondary bright fringe spacing measured between adjacent minima.

9. C — Closely spaced tracks act as a diffraction grating.

10. D — Diffraction limits the resolving power of an optical microscope.

11. True — Light can spread into a geometrical shadow region.

12. False — Longer wavelengths generally produce larger diffraction angles for the same slit width.

13. True — Diffraction patterns arise from the interference of wavelets.

14. False — The central maximum is generally twice as wide as each adjacent interval between minima.

15. True — Fraunhofer diffraction is the far-field regime.

16. A — sin θ = λ/a = (6 × 10⁻⁷)/(2 × 10⁻⁴) = 0.003.

17. B — sin θ = (5 × 10⁻⁷)/(1 × 10⁻⁴) = 0.005.

18. B — Angular width is proportional to wavelength divided by slit width, so halving the width doubles it.

19. A — sin θ = (5 × 10⁻⁷)/(2.5 × 10⁻⁴) = 0.002; for small angles, θ ≈ 0.002 rad.

20. D — Decreasing slit width increases the angular width of the central maximum.

📌 Note: Numerical answers use the single-slit diffraction equation. Small-angle approximations are used where appropriate.