Q: What does Diffraction Gratings: Spectra Analysis for A-Level Physics cover? A: Transform old CDs and DVDs into precision spectrometers.
TL;DR That scratched DVD in your drawer has a 0.74µm track pitch (~1350 lines/mm) - close to the commercial 1200-1400 lines/mm gratings exam questions reference, per the Blu-ray Disc Association physical-format spec. Calibrate it with a known laser to get few-nanometre agreement, then use gas discharge lines (NIST values for H, Hg, Na) to read unknown spectra and see why streetlights glow orange. Master the grating equation and you can predict the order angles and resolving power asked for in H2 Physics.
Why Gratings Beat Prisms
Prisms spread light by refraction - different wavelengths bend different amounts. But diffraction gratings use interference from thousands of tiny slits to create:
Higher resolution (sharper spectral lines)
Linear dispersion (even spacing)
Multiple orders (repeated spectra)
Predictable angles (via grating equation)
Your CD spectrometer will outperform Newton's prism in every measure.
The Physics Behind the Rainbow
The Grating Equation
For constructive interference at angle θ:
dsinθ=nλ
Where:
d = Slit spacing (distance between lines)
θ = Angle from normal
n = Order number (0,±1,±2...)
λ = Wavelength
Why CDs and DVDs Work
CD specifications: Track pitch 1.6µm ≈ 625 lines/mm (IEC 60908, summarised in Compact disc). DVD specifications: Track pitch 0.74µm ≈ 1350 lines/mm (Blu-ray Disc Association physical format whitepaper, see Blu-ray technical section). Blu-ray: Track pitch 0.32µm (higher resolution; same source).
Benefits: Digital recording, intensity plots, easy sharing
Measuring Known Sources
LED Wavelengths
Single-color LEDs are nearly monochromatic when you use the wavelength stated on the datasheet.
Read the dominant λ from the datasheet (manufacturers list it for each color bin).
Aim LED at slit.
View through grating.
Measure angle to first order (n=1).
Calculate: λ=dsinθ and compare with the datasheet spec.
White LED Spectrum
The US Department of Energy notes that white LEDs are typically a blue pump plus a yellow phosphor conversion layer; you should see a blue peak (≈450nm) and a broad yellow band with reduced blue-green intensity (DOE SSL LED basics).
Calculating Grating Spacing
Using Known Wavelength
With laser pointer (known λ):
Measure angles for multiple orders
Plot sinθ vs n
Gradient = dλ
Calculate d
Example with red laser (manufacturer-rated 650nm diode):
First order at 25.4°
d=sin(25.4°)650×10−9=1.52×10−6 m
Lines/mm = 1.52×10−31=658
Cross-Verification
Test your calculated d value:
Use different laser color
Predict angle using your d
Measure actual angle
Should agree within 2%
Analyzing Gas Discharge Spectra
Hydrogen Spectrum
The quantum mechanics showcase:
Visible lines (NIST ASD H I values):
Red: 656.28nm (H-alpha)
Blue-green: 486.13nm (H-beta)
Blue: 434.05nm (H-gamma)
Violet: 410.17nm (H-delta)
What you'll observe:
Discrete lines (not continuous)
Exact wavelengths match theory
Balmer series demonstration
Mercury Vapor (Fluorescent Lights)
Characteristic lines (NIST ASD Hg I strong lines):
Violet: 404.66nm
Blue: 435.83nm
Green: 546.07nm (brightest)
Yellow doublet: 576.96nm, 579.07nm
Environmental note: These narrow lines are why older fluorescent lamps distort colour rendering.
Need more lab write-ups to reinforce these spectroscopy techniques? Browse our H2 Physics experiments hub for diffraction grids, double-slit, and capacitor investigations you can adapt for promos and the H2 practical paper.
Sodium Street Lamps
The classic doublet (NIST ASD Na I D-lines):
588.995nm and 589.592nm
Usually appears as single orange line
Resolving needs R≳1000
Experimental Procedures
Calibration First
Use laser pointer (known wavelength)
Find grating constantd
Check with second laser color
Now measure unknowns
For Each Light Source
Ensure stable setup (no movement)
Align carefully (perpendicular incidence)
Measure both sides (±n orders)
Average for accuracy
Record order number clearly
Data Collection Table
Source
Order
θ(∘)
sinθ
λ(nm)
Color
Red LED
+1
24.8
0.420
638
Red
Red LED
-1
-24.9
-0.421
639
Red
Red LED
+2
54.6
0.815
619
Red †
†: 2nd-order measurement is less accurate at large diffraction angles. At those angles the diffracted beam is weaker, more dispersed, and can overlap with other orders, all of which reduce wavelength-measurement precision
At 30∘: sinθδ(sinθ)≈1.5% → ±9–10nm uncertainty near 650nm
Improves with digital angle sensor or camera fitting (0.1∘ resolution gives few-nanometre agreement)
Systematic Errors
Common issues:
Grating not perpendicular (shifts all angles)
Wide slit (broadens lines)
Stray light (false readings)
Temperature effects (minimal for gratings)
Improving Accuracy
Use higher orders when possible
Average multiple measurements
Measure complementary angles
Digital photography for angle analysis
Connecting to Atomic Physics
Energy Levels and Photons
Each spectral line represents:
E=hf=λhc
For hydrogen red line (656nm):E=656×10−96.63×10−34×3×108=3.03×10−19 J=1.89 eV
This matches n=3 to n=2 transition perfectly!
Spectroscopy Applications
Your simple setup demonstrates:
Element identification (forensics)
Star composition analysis
LED quality testing
Laser wavelength verification
Common Exam Questions
Q1: "Why are higher orders dimmer?"
Key points:
Light energy spread among orders
Path differences increase
Not all rays interfere constructively
Blazed gratings optimize one order
Q2: "Calculate wavelength from measurements"
Given:600 lines/mm grating, first order at 22.5∘
Solution:
d=6001=1.67×10−6m
λ=dsinθ=1.67×10−6×sin(22.5∘)
λ=639nm (red light)
Q3: "Why do CDs show colors but mirrors don't?"
Model answer:
CDs have regular line spacing (d≈λ)
Acts as diffraction grating
Different wavelengths diffract at different angles
Mirrors have no regular structure
Practical Applications
DIY Projects
Gem testing: Real vs synthetic by fluorescence
Plant health: Chlorophyll absorption spectra
Monitor calibration: Check RGB peaks
Chemistry: Flame test spectra
Industry Uses
Quality control: LED manufacturing
Environmental: Pollution monitoring
Medical: Blood oxygen sensors
Astronomy: Exoplanet detection
Building a Quantitative Spectrometer
Professional-Grade Features
For research-quality results:
Collimating lens (parallel light)
Focusing lens (sharp image)
Micrometer slit (adjustable width)
Rotation stage (precise angles)
Digital Detection
Replace eye with:
Webcam (remove IR filter)
DSLR in RAW mode
Specialized sensor (linear CCD)
Software for calibration
Wavelength Calibration
Using multiple known sources:
Plot pixel position vs wavelength
Fit polynomial curve
Interpolate unknown wavelengths
Achieve few-nanometre accuracy once the angle fit is tied to multiple reference lines
Your Laboratory Checklist
✓ Remove metallic layer from CD/DVD carefully ✓ Make slit narrow (0.5mm or less) ✓ Work in darkened room for best contrast ✓ Calibrate with laser first ✓ Measure angles from perpendicular ✓ Check both positive and negative orders ✓ Calculate wavelengths using dsinθ=nλ ✓ Compare with published values
Master diffraction gratings and you hold the key to understanding atomic structure, analyzing starlight, and checking if that "ruby" is real. From forensic labs to semiconductor fabs, spectroscopy rules - and it all starts with the physics you're exploring here.
