diff --git a/src/objects/sphere.rs b/src/objects/sphere.rs
index 79f464a..4f36ee1 100644
--- a/src/objects/sphere.rs
+++ b/src/objects/sphere.rs
@@ -21,16 +21,15 @@ impl Sphere {
 impl Hittable for Sphere {
     fn hit(&self, r: &crate::ray::Ray) -> Option<Hit> {
         let oc = self.center - r.origin();
-        let a = r.dir().dot(r.dir());
-        let b = -2. * r.dir().dot(&oc);
-        let c = oc.dot(&oc) - self.radius * self.radius;
-
-        let d = b * b - 4. * a * c;
+        let a = r.dir().length_squared();
+        let h = r.dir().dot(&oc);
+        let c = oc.length_squared() - self.radius * self.radius;
+        let d = h * h - a * c;
 
         if d < 0. {
             None
         } else {
-            let t = (-b - sqrt(d)) / (2.0 * a);
+            let t = (h - sqrt(d)) / a;
             let p = r.at(t);
             Some(Hit::new(t, p, self.normal_at(&p)))
         }