physics: Add malus law

physics/src/lib.rs

@@ -11,6 +11,7 @@
 )]
 
 pub mod kinetic_energy;
+pub mod malus_law;
 pub mod potential_energy;
 pub mod shear_stress;
 pub mod speed_of_sound;

physics/src/malus_law.rs

@@ -0,0 +1,36 @@
+// Copyright (c) 2023 Xu Shaohua <shaohua@biofan.org>. All rights reserved.
+// Use of this source is governed by General Public License that can be found
+// in the LICENSE file.
+
+//! Finding the intensity of light transmitted through a polariser using Malus Law.
+//!
+//! [Malus's law](https://en.wikipedia.org/wiki/Polarizer#Malus's_law_and_other_properties)
+
+#[must_use]
+pub fn malus_law(initial_intensity: f64, angle: f64) -> f64 {
+    debug_assert!(initial_intensity >= 0.0);
+    debug_assert!((0.0..=360.0).contains(&angle));
+    initial_intensity * (angle.to_radians().cos()).powi(2)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::malus_law;
+
+    #[test]
+    fn test_malus_law() {
+        const PAIRS: &[(f64, f64, f64)] = &[
+            (10.0, 45.0, 5.0),
+            (100.0, 60.0, 25.0),
+            (50.0, 150.0, 37.5),
+            (75.0, 270.0, 0.0),
+            (100.0, 180.0, 100.0),
+            (100.0, 360.0, 100.0),
+        ];
+        for (initial_intensity, angle, result) in PAIRS {
+            let intensity = malus_law(*initial_intensity, *angle);
+            let round_intensity = (intensity * 10.0).round() / 10.0;
+            assert_eq!(round_intensity, *result);
+        }
+    }
+}