diff --git a/frontend/js/labs/opticsbench.js b/frontend/js/labs/opticsbench.js
index 7a8b258..0181e1b 100644
--- a/frontend/js/labs/opticsbench.js
+++ b/frontend/js/labs/opticsbench.js
@@ -2616,14 +2616,16 @@ class PrismSim {
     const eX = (v[0].x + v[2].x) / 2, eY = (v[0].y + v[2].y) / 2;
     const tangDir = { x: efVec.x / efLen, y: efVec.y / efLen };
     const incRad  = this.incAngle * Math.PI / 180;
+    /* incDir = propagation direction (INTO the prism) */
     const incDir  = {
-      x: Math.cos(incRad) * (-efNorm.x) + Math.sin(incRad) * tangDir.x,
-      y: Math.cos(incRad) * (-efNorm.y) + Math.sin(incRad) * tangDir.y,
+      x: Math.cos(incRad) * efNorm.x + Math.sin(incRad) * tangDir.x,
+      y: Math.cos(incRad) * efNorm.y + Math.sin(incRad) * tangDir.y,
     };
     const incDLen = Math.hypot(incDir.x, incDir.y);
     incDir.x /= incDLen; incDir.y /= incDLen;
 
     const rayLen = W * 0.45;
+    /* draw incoming ray from outside (source side) to entry midpoint */
     this._drawRayLine(ctx, eX - incDir.x * rayLen, eY - incDir.y * rayLen, eX, eY,
       isWhite ? '#FFFFFF' : wavelengthToRGB(monoNm), 2.5);
 
@@ -2639,13 +2641,15 @@ class PrismSim {
     for (const s of spectral) {
       const nP  = _nAtWavelength(this.n0, s.nm);
       const col = wavelengthToRGB(s.nm);
-      const cosI = -(incDir.x * efNorm.x + incDir.y * efNorm.y);
+      /* Snell vector form: l = incDir (propagation), n = -efNorm (toward incident medium)
+         cosθ_i = -n·l = efNorm·l;  r = μl + (μcosθ_i - cosθ_t)·n */
+      const cosI = (incDir.x * efNorm.x + incDir.y * efNorm.y);
       const sinR = Math.sqrt(Math.max(0, 1 - cosI * cosI)) / nP;
       if (sinR > 1) continue;
       const cosR = Math.sqrt(1 - sinR * sinR);
       const rDir = {
-        x: (1/nP) * incDir.x + ((1/nP) * cosI - cosR) * efNorm.x,
-        y: (1/nP) * incDir.y + ((1/nP) * cosI - cosR) * efNorm.y,
+        x: (1/nP) * incDir.x + (cosR - (1/nP) * cosI) * efNorm.x,
+        y: (1/nP) * incDir.y + (cosR - (1/nP) * cosI) * efNorm.y,
       };
       const rDLen = Math.hypot(rDir.x, rDir.y);
       rDir.x /= rDLen; rDir.y /= rDLen;