From 316def4ea65f68ca8982148b97896c8bfef27e87 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20S=C3=A1nchez=20N=C3=BA=C3=B1ez?=
 <dsannue@upv.edu.es>
Date: Sun, 7 Dec 2025 12:18:55 +0100
Subject: [PATCH] Add Snell's Law refraction algorithm (#7150)

* Add Snell's Law implementation for refraction angle

* Add tests for Snell's Law calculations

* Update documentation with reference link to Snell's Law

Added a reference link to Snell's Law in the documentation.

* Prevent instantiation of SnellLaw class

Make SnellLaw class non-instantiable by adding a private constructor.

* Rename SnellsLawTest to SnellLawTest

* Refactor SnellLawTest for clarity and accuracy

* Rename SnellsLaw.java to SnellLaw.java

* Refactor SnellLawTest with additional assertions

* Refactor SnellLaw class constructor and error handling

Refactor SnellLaw constructor and error message formatting.

* Fix missing newline at end of SnellLawTest.java

Ensure that the SnellLawTest class has a newline at the end of the file.

* Simplify assertions in SnellLawTest

* Simplify exception throwing for total internal reflection
---
 .../com/thealgorithms/physics/SnellLaw.java   | 33 +++++++++++++++
 .../thealgorithms/physics/SnellLawTest.java   | 41 +++++++++++++++++++
 2 files changed, 74 insertions(+)
 create mode 100644 src/main/java/com/thealgorithms/physics/SnellLaw.java
 create mode 100644 src/test/java/com/thealgorithms/physics/SnellLawTest.java

diff --git a/src/main/java/com/thealgorithms/physics/SnellLaw.java b/src/main/java/com/thealgorithms/physics/SnellLaw.java
new file mode 100644
index 000000000..273698481
--- /dev/null
+++ b/src/main/java/com/thealgorithms/physics/SnellLaw.java
@@ -0,0 +1,33 @@
+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);
+    }
+}
diff --git a/src/test/java/com/thealgorithms/physics/SnellLawTest.java b/src/test/java/com/thealgorithms/physics/SnellLawTest.java
new file mode 100644
index 000000000..ddd5fb1d5
--- /dev/null
+++ b/src/test/java/com/thealgorithms/physics/SnellLawTest.java
@@ -0,0 +1,41 @@
+package com.thealgorithms.physics;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+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() {
+        double n1 = 1.5;
+        double n2 = 1.0;
+        double theta1 = Math.toRadians(60); // large 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));
+    }
+}