Optics
Diffraction grating
Many slits sharpen the maxima into thin bright orders at d·sinθ = mλ — the basis of a spectrometer.
Diffraction grating — interactive Optics simulation. Many slits sharpen the maxima into thin bright orders at d·sinθ = mλ — the basis of a spectrometer. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Diffraction grating
Many slits sharpen the maxima into thin bright orders at d·sinθ = mλ — the basis of a spectrometer.
N parallel slits interfere constructively at angles satisfying d sin θ = mλ, concentrating energy into sharp spectral orders. Higher line density d improves wavelength resolution—why gratings replace prisms in precision spectrometers.
- d sin θ = mλ
- Sharp orders
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
A diffraction grating has hundreds of tightly-spaced parallel slits (measured in lines per millimeter). Does a grating with MORE lines per millimeter diffract light to a larger or smaller angle for the first bright peak?
Predictions to weigh
- Larger angle — more lines per mm means a smaller spacing between slits, which spreads the diffraction pattern to a wider angle.
- A denser grating gives a smaller diffraction angle.
- The diffraction angle doesn't depend on lines per mm.
Variable roles
What you set:
- Lines per mm
What you measure:
- First-order angle θ (°)
How the investigation runs
- Open the grating preset and press Reset. Light wavelength is 500 nm.
- Enable the first-order diffraction angle readout.
- Set the grating's line density for each trial and record the first-order bright-fringe angle.
Governing equation
Diffraction Grating Equation — d·sinθ = mλ
A grating with slit spacing d produces bright fringes wherever d·sin θ = mλ (m = 1, 2, 3... for successive orders). A denser grating (smaller d, more lines per mm) diffracts a given wavelength to a larger angle.
What the printable worksheet asks students to work out
- For one trial, compute the slit spacing d = 1/(lines per mm) in mm, then solve d·sin θ = λ for θ, using λ = 500 nm (converting units carefully). Compare to the table.
- Explain why a HIGHER line density (more lines per mm) means a SMALLER slit spacing d, which — since d·sinθ = λ — requires a LARGER angle θ to satisfy the same wavelength condition.
Where this shows up beyond the lab
- Diffraction gratings are the heart of most spectrometers: different wavelengths diffract to different angles from the same grating, spatially separating a light source's spectrum for analysis. Explain why a denser grating (more lines per mm) gives BETTER wavelength resolution — it spreads a given wavelength range over a wider angular range.
- The colorful rainbow you see reflecting off a CD or DVD is diffraction grating behavior — the disc's closely-spaced data tracks act like a reflection grating. Explain why tilting a CD at different angles reveals different colors most brightly.
- AP Physics 2 — Unit 14: Waves, Sound, and Physical Optics
- IB Physics — C.3 Wave phenomena
- General High School Physics — Light & optics
- NGSS High School Physics — Wave properties
- Welcome to Grating
- Select the grating
- Press Play
- Sharp spectral orders
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.