PHY021 · Optics · Class 12 + College
Diffraction grating
Objective: To measure wavelengths of light using a diffraction grating.
🧰 Apparatus
- Diffraction grating (e.g. 600 lines/mm)
- Spectrometer or laser and screen
- Metre scale
📐 Formulas and SI units
d sin θ = mλ, d = 1/N (N lines per metre)
💡 Theory
A grating has many equally spaced slits. Light from all of them reinforces only in directions where d sin θ = mλ, giving sharp bright orders. More lines per mm spread the orders further apart.
🪜 Procedure
- Mount the grating perpendicular to the beam.
- Find the central bright line (m = 0).
- Measure the angle to the first-order line on both sides.
- Average the two and calculate λ = d sin θ / m.
- Repeat with the second order and with another colour.
📋 Observations Simulated values
✅ Result and conclusion
For 600 lines/mm and λ = 500 nm the first order is at 17.46°; at most three orders are visible.
🔬 In a real lab
Orders are not perfectly sharp; spectrometer verniers read to about 1′. Gratings also give weaker secondary maxima.
⚠️ Safety
Do not look directly into lamps or lasers; mercury and sodium lamps get hot.
❓ Quick check
1. More lines per mm make the first-order angle…
2. The central order m = 0 is…
3. Which deviates more in the first order: red or violet?
🏋️ Practice
Easyd = 2 µm, λ = 600 nm, m = 1. θ?
Answer: ≈ 17.5°
MediumMaximum order for d = 1.67 µm, λ = 500 nm?
Answer: 3
Challengeθ₁ = 30° for d = 1.2 µm. λ?
Answer: 600 nm