Add tests for Snell's Law calculations

Author: Dsannue

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

@@ -0,0 +1,32 @@
+package com.thealgorithms.physics;
+
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+
+public class SnellsLawTest {
+//tests for example environmet
+    @Test
+    public void testCalculateRefractionAngle() {
+        double n1 = 1.0;  // air
+        double n2 = 1.5;  // glass
+        double theta1 = Math.toRadians(30); // 30 grados
+        
+        double theta2 = SnellsLaw.calculateRefractionAngle(n1, n2, theta1);
+        
+        // expected value using: sin(theta2) = n1/n2 * sin(theta1)
+        double expected = Math.asin((n1 / n2) * Math.sin(theta1));
+
+        assertEquals(expected, theta2, 1e-9);
+    }
+//tests for total refraction
+    @Test
+    public void testInvalidRefractiveIndex() {
+        assertThrows(IllegalArgumentException.class, () -> {
+            SnellsLaw.calculateRefractionAngle(-1, 1.5, 0.5);
+        });
+
+        assertThrows(IllegalArgumentException.class, () -> {
+            SnellsLaw.calculateRefractionAngle(1, 0, 0.5);
+        });
+    }
+}