Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

Theme C — Wave Behaviour

C.3.9 Diffraction Gratings — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 25 of 37 1 quiz

📚 IB Physics HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read
Lesson summary

C.3.9 Diffraction Gratings — IB PHYSICS HL notes: Use the diffraction grating equation nλ = d sin θ to find angles, orders and wavelengths and describe the pattern from white light.Structure — many very thin, equally spaced parallel slits cut into a glass plateWith monochromatic …

  • With monochromatic light
  • With white light
  • Spectrometers
  • What the symbols mean
  • Zeroth order
  • Lines per unit length

Part 1 of 8The diffraction grating

The diffraction grating
Structure
  • Structure many very thin, equally spaced parallel slits cut into a glass plate
With monochromatic light
  • With monochromatic light sharp bright and dark fringes
With white light
  • With white light the light is split into its wavelength components
Spectrometers
  • Spectrometers gratings analyse light from stars and gas discharge lamps

Part 2 of 8Grating equation

Equation
  • Equation nλ = d sin θ
    What the symbols mean: d = grating spacing (m) · θ = angle of the nth-order maximum · n = order number (0, 1, 2 …) · λ = wavelength (m)
n
n the order of the maximum, counted from the central maximum (n = 0)
d
d the distance between adjacent slits in metres
θ
θ the angle of the order from the straight-through direction
λ
λ the wavelength of the light
Symmetry
  • Symmetry orders appear on both sides of the central beam
Zeroth order
  • Zeroth order the undeviated central beam at θ = 0 for all wavelengths

Part 3 of 8Slit spacing

Lines per unit length
  • N is the number of lines per mm or per m
Spacing
  • Spacing d = 1/N
  • N in lines per metre gives d in metres
Conversion
  • 600 lines per mm gives d = 1/600 mm = 1.67 × 10−6 m

Part 4 of 8Sharp, bright maxima

Comparison
  • Comparison a grating gives narrower, brighter fringes and wider, darker gaps than a double slit
Reason
  • Reason away from an exact maximum the contributions of many slits cancel
More slits
  • More slits more and brighter maxima and sharper lines
Precision
  • Precision sharp lines allow accurate angle and wavelength measurements

Part 5 of 8Angles and angular separation

Angle of an order
  • θ = sin−1(nλ/d)
Angular separation
  • Angular separation subtract the smaller angle from the larger
Higher orders
  • Higher orders they lie at greater angles from the centre
Measurement
  • Measurement angles are measured from the central maximum and not between two orders

Part 6 of 8Highest order

Limit
  • Limit the largest angle is 90° so sin θ = 1
Equation
  • Equation nmax = d/λ
Rounding
  • Rounding round down to a whole number
Total lines
  • 2nmax + 1 including both sides and the central maximum

Part 7 of 8White light

Central maximum
  • Central maximum a thin white strip because all wavelengths interfere constructively there
Other orders
  • Other orders each is a spectrum
Violet or blue
  • Violet or blue the shortest wavelength is diffracted least, nearest the centre
Red
  • Red the longest wavelength is diffracted most, furthest from the centre
Overlap
  • Overlap higher orders spread more and overlap where n1λ1 = n2λ2
Analysis
  • Analysis the first-order spectrum is used
Compared with a prism
  • Compared with a prism a grating deviates red most while a prism deviates violet most

Part 8 of 8Choosing a grating

Smaller spacing
  • Smaller spacing more lines per mm gives larger angles for each order and fewer orders
Higher resolution
  • Higher resolution a higher order or more lines per mm separates close wavelengths better
Beam
  • Beam a narrow collimated beam allows angles to be measured without blurring

Key terms

TermMeaning
Diffraction gratingMany equally spaced slits producing sharp interference maxima.
Order (n)The integer labelling each maximum, counted from the central maximum.
Grating spacing (d)The distance between adjacent slits, d = 1/N.
Zeroth orderThe undeviated central maximum.
SpectrometerAn instrument that uses a grating to separate wavelengths.

Avoid these mistakes

1
Using the number of lines per mm directly as d — invert it and convert to metres.
2
Counting only one side when finding the total number of maxima.
3
Rounding d/λ up instead of down to find the maximum order.
4
Thinking violet is deviated more than red by a grating — red is deviated more.
5
Forgetting the zeroth order when counting lines.

Last updated:

\n