Add Snell's Law refraction algorithm

src/main/java/com/thealgorithms/physics/SnellsLaw.java

@@ -0,0 +1,31 @@
+package com.thealgorithms.physics;
+
+/**
+ * Calculates refraction angle using Snell's Law:
+ * n1 * sin(theta1) = n2 * sin(theta2)
+ * @see <a href="https://en.wikipedia.org/wiki/Snell%27s_law">Snell's Law</a>
+ */
+public final class SnellLaw {
+     private SnellLaw() {
+        throw new AssertionError("No instances.");
+    }
+    /**
+     * Computes the refracted angle (theta2) in radians.
+     *
+     * @param n1 index of refraction of medium 1
+     * @param n2 index of refraction of medium 2
+     * @param theta1 incident angle in radians
+     * @return refracted angle (theta2) in radians
+     * @throws IllegalArgumentException if total internal reflection occurs
+     */
+    public static double refractedAngle(double n1, double n2, double theta1) {
+        double ratio = n1 / n2;
+        double sinTheta2 = ratio * Math.sin(theta1);
+
+        if (Math.abs(sinTheta2) > 1.0) {
+            throw new IllegalArgumentException("Total internal reflection: no refraction possible.");
+        }
+
+        return Math.asin(sinTheta2);
+    }
+}

src/test/java/com/thealgorithms/physics/SnellLawTest.java

@@ -0,0 +1,43 @@
+package com.thealgorithms.physics;
+
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+
+public class SnellLawTest {
+
+    @Test
+    public void testRefractedAngle() {
+        double n1 = 1.0;  // air
+        double n2 = 1.5;  // glass
+        double theta1 = Math.toRadians(30);
+
+        double theta2 = SnellLaw.refractedAngle(n1, n2, theta1);
+
+        double expected = Math.asin((n1 / n2) * Math.sin(theta1));
+
+        assertEquals(expected, theta2, 1e-12);
+    }
+
+    @Test
+    public void testTotalInternalReflection() {
+        // total internal reflection happens when n1 > n2 AND theta1 is large
+        double n1 = 1.5;
+        double n2 = 1.0;
+        double theta1 = Math.toRadians(60); // large enough angle
+
+        assertThrows(IllegalArgumentException.class, () -> {
+            SnellLaw.refractedAngle(n1, n2, theta1);
+        });
+    }
+
+    @Test
+    public void testNoTotalInternalReflectionAtLowAngles() {
+        double n1 = 1.5;
+        double n2 = 1.0;
+        double theta1 = Math.toRadians(10);
+
+        assertDoesNotThrow(() -> {
+            SnellLaw.refractedAngle(n1, n2, theta1);
+        });
+    }
+}