NEB Class 12 Physics Diffraction of Light Notes in PDF Complete Handwritten. Physics Notes 2082: All Chapters | New Curriculum | Class 12 Physics Notes download.
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NEB Class 12 Physics Diffraction of Light Note Handwritten in PDF
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What is Diffraction?
Diffraction is the bending of waves around obstacles and spreading through narrow openings. It occurs for all waves sound, water, light, X-rays.
Key condition: Diffraction is most significant when slit width (a) ≈ wavelength (λ)
- If a >> λ: Almost no diffraction (geometric shadow, straight rays)
- If a ≈ λ: Significant diffraction (waves spread widely)
- If a < λ: Waves spread in all directions
Why sound diffracts easily around buildings but light doesn’t:
- Sound wavelength: few cm to metres → comparable to building doors and windows
- Light wavelength: ~500 nm → buildings are billions of times larger than λ
Single Slit Diffraction Pattern
When monochromatic light passes through a single slit of width a:
Pattern on screen:
- Central maximum — brightest, widest, centered at straight-through position
- Secondary maxima — on both sides of center, getting dimmer and narrower
- Dark fringes (minima) between the maxima
Condition for dark fringes (minima): a sinθₙ = nλ where n = ±1, ±2, ±3…
Note: This formula gives MINIMA for single slit (opposite to double slit where nλ gives maxima a common exam trap!)
Width of central maximum:
- Angular width = 2λ/a (from −1st to +1st minimum)
- Linear width on screen = 2λD/a
Effect of narrowing the slit: Wider central maximum (a decreases → 2λ/a increases). This is counterintuitive but correct narrower slit produces wider diffraction pattern.
Diffraction Grating
A diffraction grating consists of thousands of equally spaced parallel slits ruled on glass or metal.
- Grating element (d): Distance between adjacent slits = 1/N (N = number of slits per metre)
- Typical gratings have 500–6000 lines per mm
Grating equation: d sinθ = nλ where n = 0, ±1, ±2… (order of maximum)
- n = 0: Central maximum (all wavelengths overlap → white light here)
- n = 1: First order spectrum on both sides of center
- n = 2: Second order spectrum (wider, dimmer)
Finding wavelength using grating: λ = d sinθ/n → Measure θ for known n and d, calculate λ
This is a standard NEB practical and numerical. Grating gives very sharp, bright maxima (unlike double slit) useful for precise wavelength measurement.
Resolving Power of Optical Instruments
Resolving power is the ability of an optical instrument to distinguish between two closely spaced objects or spectral lines as separate.
Rayleigh’s Criterion: Two point sources are just resolved when the central maximum of one falls on the first minimum of the other.
Minimum angle of resolution for circular aperture: θ_min = 1.22λ/D (D = diameter of aperture)
Implications:
- Larger aperture (D) → smaller θ_min → better resolution
- Smaller wavelength (λ) → better resolution
- This is why large telescope mirrors resolve stars better than small ones
- This is why electron microscopes (very small λ) resolve much finer details than light microscopes
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Frequently Asked Questions
What is diffraction of light in Class 12 Physics?
Diffraction is bending and spreading of light around obstacles or through narrow openings. Most pronounced when slit width (a) is comparable to wavelength (λ). Sound diffracts easily around buildings because its wavelength is comparable to openings. Light requires very narrow slits to show noticeable diffraction.
What is diffraction grating formula Class 12?
Diffraction grating formula: d sinθ = nλ. Where d is grating element (1/N, N = lines per metre), θ is angle of diffraction, n is order of maximum (0, ±1, ±2), and λ is wavelength. To find wavelength: λ = d sinθ/n. Measure θ for known n and d experimentally then calculate λ accurately.
What is the condition for single slit diffraction minima?
Position of dark fringes (minima) in single slit diffraction: a sinθ = nλ, where a is slit width and n = ±1, ±2, ±3. Important: this formula gives MINIMA for single slit but MAXIMA for double slit interference a very common point of confusion in NEB exams. Always remember which applies to which situation.
What is Rayleigh’s criterion for resolving power?
Rayleigh’s criterion: two point sources are just resolved when central maximum of one’s diffraction pattern falls on first minimum of the other. Minimum angle of resolution: θ_min = 1.22λ/D, where D is aperture diameter. Larger aperture gives smaller θ_min and better resolution reason large telescope mirrors resolve faint stars.
What is the difference between diffraction and interference?
Interference occurs between waves from a few discrete separate sources (two slits). Diffraction occurs from different parts of the same wavefront bending through one slit. Interference fringes have equal intensity. Diffraction fringes are unequal central maximum is much brighter and wider than secondary maxima on either side.
What happens to diffraction pattern when slit is narrowed?
Narrowing the slit (decreasing a) makes central maximum WIDER: width = 2λD/a. This is counterintuitive smaller opening spreads light more. When a >> λ almost no diffraction occurs (straight rays). When a ≈ λ maximum diffraction occurs. This is why doorways diffract sound but not visible light at all.
How does diffraction grating work in Class 12 Physics?
A diffraction grating has thousands of equally spaced parallel slits. Light from all slits diffracts and interferes. Constructive interference occurs at specific angles given by d sinθ = nλ. Different wavelengths diffract at different angles grating separates white light into its spectrum more sharply than a prism does.
Which diffraction topics are most important for Exam?
Most important: diffraction grating formula with numericals (4 marks), single slit diffraction minima condition (2 marks), Rayleigh’s criterion for resolution (2 marks), difference between diffraction and interference (2 marks). Diffraction grating wavelength calculation numericals appear in almost every NEB Class 12 board paper.