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Spectral Horn
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About the game
Spectral Horn asks what would happen if Newton had used a unicorn horn instead of a prism. You steer the light of a crystal horn through a small optics lab, and the goal on every level is to charge the sensors. Each sensor demands a specific colour, so it matters not only where the beam goes but what colour it is when it arrives.
The beam is traced live with real rules: Snell's law for refraction, a wavelength-dependent refractive index that splits white light into a spectrum, total internal reflection, and mirrors. Along the way you meet Dove prisms that flip the order of the colours without bending the beam, crystal orbs that focus diverging light, and levels where overlapping red and green light makes yellow, or a second horn recombines the spectrum into white. Hold a correct beam steady until a sensor reaches 100 per cent and it locks.
Filip Paulů made it for js13kGames 2026, where the whole game had to fit in 13 KB. It has seventeen levels and a sandbox, and the pictures, the music and the sound effects are all generated by the code.
Playing it here
Click Play on the title screen. Drag the unicorn to move it, drag the ring around its horn to rotate it, and use the small arrow buttons for precise steps. The top bar has a level selector, Reset if you are stuck, Help with the rules of each piece, and switches for music and sound effects. The credit "Made by Filip" in the corner opens the author's site in another tab. Your progress is not saved between visits.
Controls
| Input | Action |
|---|---|
| drag the unicorn | move it |
| drag the ring around the horn | rotate the horn |
| step buttons (arrows) | rotate by 0.5 degrees |
| Menu, Reset, Help | top bar buttons |
| level selector | jump to any unlocked level |
| Music / SFX | sound switches |
| touch | the same, with a finger |
A few things worth knowing
Read the sensor colour first. A sensor asks for a specific wavelength, so a beam that arrives in the wrong colour does nothing.
Use the small steps for the last degree. A fraction of a degree can be the difference between hitting a sensor and missing it.
Keep it steady. A sensor only locks when the right beam stays on it long enough to charge.
Credits and licence
Spectral Horn was created by Filip Paulů and is released under the MIT licence; the licence text is linked from this page.