Here is an app that simulates the interference pattern projected onto a wall or screen from an LED laser shining through 4 diffraction gratings.
Click on the below image of the app to run it in a separate window. I suggest trying a “Max Diffraction Order” of 2 (the bottom slider). The “Pair Rotation Angle” (top slider) can be anywhere from 0° to 90°. Values around 60° or so forn inages that look pretty cool to me. I especially like the way the image changes as you step through the angles 60°, 61°, 62°.

This laser interference pattern is similar to what would be produced by a Laser pointer’s beam passing through a diffraction grating – When you shine a laser pointer (green, 532nm wavelength) through a series of transmission diffraction gratings, you get a beautiful regular array of bright spots. The center spot is the zero-order (undiffracted beam), and the surrounding spots are the various diffraction orders. The symmetry and regularity of the pattern is characteristic of a diffraction grating pattern.
This simulation could be used as a demonstration setup for physics education – teaching about wave optics, interference and diffraction. It is a classic physics demonstration that beautifully illustrates the wave nature of light and the principles of diffraction.
Note that:
- The center spot is brightest (zero-order maximum)
- There’s symmetry in the pattern
- The spots gradually dim as you move away from center, but not in a trivial way
- It’s distinctly green (characteristic of common 532nm laser pointers)
The image produced (and simulated in the app) resembles a green laser pointer shining through a diffraction grating onto a screen or wall.
Here is a YouTube video of the app being used to interactively explore the effect of using various Rotation Angles between 0 and 90 degrees:
Additional detail:
The beam passes through 4 gratings (2 pairs of gratings), and the grating pairs are rotated relative to each other along the axis of the beam. I modeled this mathematically so I could simulate this spot pattern in software, providing the relative angle as input.
Modeling a multiple-grating system is a fascinating optics problem. The simulation accounts for both pairs of diffraction gratings and their relative rotation.
Dual Diffraction Grating Simulator
First I modeled an interactive simulator for dual-grating diffraction. Here’s how the mathematical model works:
Key Physics:
- Each grating follows the diffraction equation:
- sin(θ) = m·λ/d
- Where m is the diffraction order (integers), λ is wavelength, d is grating pitch
- Dual grating approach:
- First grating creates diffraction orders in one direction
- Second grating (rotated by angle α) creates its own diffraction pattern
- Each spot from the first grating acts as a source for the second grating
- Final pattern is the superposition of all combinations
- Position calculation:
- Each spot position combines contributions from both gratings
- The second grating’s contribution is rotated by angle α
- This creates the complex 2D patterns you observed
Try adjusting:
- Rotation angle (0-90°): Causes the pattern to evolve from a simple 1D grid (row of spots) to a 2D grid arrangement.
- Grating pitch: Changes the spacing between spots
- Max order: Controls how many diffraction orders are included
That simulator was a good start, but I wanted it to show more complex behavior like I’ve seen generated by a laser pointer I bought from a pawn shop several years ago. It appeared to me that there must be more going on with that actual laser pointer (see photos below) than just combining two gratings. So I made a guess that there are two pairs of gratings (4 gratings in total, in 2 groups of 2), and the first pair is rotated relative to the second pair.
Here is the laser pointer, sitting flat and then about to be mounted on a tri-pod:


And here (below) is the complex dot image projected on a wall. This is what originally inspired me to try and simulate it in software.

So in an effort to try and get this interesting behavior in software, I took the 2 grating model a step further, by using 4 gratings.
I’ll call the gratings G0, G1, G2 and G3.
- G0 and G1 are rotated 90 degrees relative to each other.
- G2 and G3 are rotated 90 degrees relative to each other.
The adjustable “angle” parameter now is the angle between these two pairs of gratings. This will hopefully create even more complex and beautiful interference patterns.
The simulator app as it is now uses 4 gratings. Here’s how it works:
Grating Configuration:
- G0 and G1: Fixed at 0° and 90° (perpendicular pair)
- G2 and G3: Rotated by the adjustable angle, also perpendicular to each other
- The angle slider controls the rotation between these two pairs
What the app shows:
- 0°: All gratings aligned in cardinal directions → square grid
- 22.5°: Interesting octagonal patterns
- 45°: More complexity with beautiful star/flower patterns
- Near 60°: maximum complexity (as I perceive it)
- 90°: Back to square grid (rotated)
The number of spots grows as (2·order+1)⁴, so be careful with high orders! Order 3 gives you 7⁴ or 2401 spots, which creates incredibly rich patterns, but performance may start to suffer and some unwelcome artifacts can make their way into the image.
Try sliding the angle slowly and watch how the pattern morphs – you should see fascinating symmetries emerge at different angles. This 4-grating setup creates much more complex and beautiful interference patterns than the 2-grating version.
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