diff --git a/frontend/js/labs/opticsbench.js b/frontend/js/labs/opticsbench.js
index 0181e1b..8d57a76 100644
--- a/frontend/js/labs/opticsbench.js
+++ b/frontend/js/labs/opticsbench.js
@@ -2614,7 +2614,9 @@ class PrismSim {
       efNorm = { x: -efNorm.x, y: -efNorm.y };
     }
     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 };
+    /* tangDir = 90° CW rotation of efNorm so that positive incAngle
+       tilts the ray toward the apex side (natural source-from-left setup) */
+    const tangDir = { x: efNorm.y, y: -efNorm.x };
     const incRad  = this.incAngle * Math.PI / 180;
     /* incDir = propagation direction (INTO the prism) */
     const incDir  = {
@@ -3306,6 +3308,12 @@ function obSwitchMode(mode, silent) {
     if (!prismSim) {
       const cv = document.getElementById('ob-prism-canvas');
       if (cv) prismSim = new PrismSim(cv);
+      /* enable white-light dispersion by default on first prism entry */
+      if (window._obWhiteLight === false) {
+        window._obWhiteLight = true;
+        const wlBtn = document.getElementById('ob-prism-white-btn');
+        if (wlBtn) wlBtn.classList.add('active');
+      }
     }
     if (prismSim) { prismSim.fit(); prismSim.draw(); }
     _obDrawSpectrometer();
@@ -3459,6 +3467,16 @@ function prismPreset(n0, incAngle) {
   if (prismSim) prismSim.setParams({ n0, incAngle });
 }
 
+function prismToggleWhite(on, btn) {
+  window._obWhiteLight = !!on;
+  const wb = document.getElementById('ob-prism-white-btn');
+  const mb = document.getElementById('ob-prism-mono-btn');
+  if (wb) wb.classList.toggle('active', !!on);
+  if (mb) mb.classList.toggle('active', !on);
+  if (prismSim) prismSim.draw();
+  _obDrawSpectrometer();
+}
+
 /* ── Thin Lens controls ── */
 function lensParam(name, val) {
   const v = parseFloat(val);
diff --git a/frontend/lab.html b/frontend/lab.html
index 54eb690..b73bcc6 100644
--- a/frontend/lab.html
+++ b/frontend/lab.html
@@ -3173,6 +3173,11 @@
               <input type="range" id="sl-prism-inc" min="0" max="75" step="1" value="30" oninput="prismParam('incAngle',this.value)" style="flex:1">
             </div>
             <div class="pp-hint" style="margin-bottom:8px">Тащи призму мышью: ← → вращение, &#8597; угол луча</div>
+            <div class="gp-section-title" style="margin-bottom:6px">Свет</div>
+            <div style="display:flex;gap:5px;margin-bottom:10px">
+              <button class="preset-btn active" id="ob-prism-white-btn" onclick="prismToggleWhite(true,this)" style="flex:1">Белый</button>
+              <button class="preset-btn" id="ob-prism-mono-btn" onclick="prismToggleWhite(false,this)" style="flex:1">Моно</button>
+            </div>
             <div class="gp-section-title" style="margin-bottom:6px">Пресеты</div>
             <div style="display:flex;flex-wrap:wrap;gap:4px;margin-bottom:8px">
               <button class="preset-btn" onclick="prismPreset(1.5,30)">Стекло (n=1.5)</button>