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? It's a 740 lines/mm diffraction grating worth $50 in physics catalogs. This guide shows how to measure light wavelengths to ±2nm accuracy, identify elements by their spectral fingerprints, and understand why streetlights glow orange. Master the grating equation and ace every spectrum question 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.
Benefits: Digital recording, intensity plots, easy sharing
Measuring Known Sources
LED Wavelengths
Single-color LEDs are nearly monochromatic:
Red LED: λ≈630−660 nm
Green LED: λ≈520−535 nm
Blue LED: λ≈450−475 nm
Measurement procedure:
Aim LED at slit
View through grating
Measure angle to first order (n=1)
Calculate: λ=dsinθ
White LED Spectrum
Unlike single colors, white LEDs show:
Blue peak (≈450nm) from LED chip
Broad yellow from phosphor
Gap in blue-green (explains color rendering)
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 (650nm):
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:
Red: 656.3 nm (H-alpha)
Blue-green: 486.1 nm (H-beta)
Blue: 434.0 nm (H-gamma)
Violet: 410.2 nm (H-delta)
What you'll observe:
Discrete lines (not continuous)
Exact wavelengths match theory
Balmer series demonstration
Mercury Vapor (Fluorescent Lights)
Characteristic lines:
Violet: 404.7 nm
Blue: 435.8 nm
Green: 546.1 nm (brightest)
Yellow doublet: 577.0, 579.1 nm
Environmental note: This is why fluorescent lights have odd color rendering!
Sodium Street Lamps
The classic doublet:
589.0 nm and 589.6 nm
Usually appears as single orange line
Need good grating to resolve doublet
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
\(\theta \space (^\circ)\)
\(\sin \theta\)
\(\lambda \pu{(nm)}\)
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
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 ±1nm accuracy
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.
Diffraction Gratings: Spectra Analysis for A-Level Physics
