diff --git a/src/interval.h b/src/interval.h
new file mode 100644
index 0000000..a2d0dfa
--- /dev/null
+++ b/src/interval.h
@@ -0,0 +1,24 @@
+#pragma once
+
+#include "raytracer.h"
+class Interval {
+   public:
+    static const Interval kEmpty, kFull, kPositive;
+
+    constexpr Interval() = default;
+    constexpr Interval(double min, double max) : min_{min}, max_{max} {}
+
+    constexpr double min() const { return min_; }
+    constexpr double max() const { return max_; }
+
+    constexpr bool contains(double x) const { return min_ <= x && x <= max_; }
+    constexpr bool surronds(double x) const { return min_ < x && x < max_; }
+
+   private:
+    double min_ = kInf;
+    double max_ = -kInf;
+};
+
+constexpr Interval Interval::kEmpty{kInf, -kInf};
+constexpr Interval Interval::kFull{-kInf, kInf};
+constexpr Interval Interval::kPositive{0.0, kInf};
diff --git a/src/main.cc b/src/main.cc
index 0753b14..1d1c406 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -4,6 +4,7 @@
 #include "camera.h"
 #include "colour.h"
 #include "image.h"
+#include "interval.h"
 #include "ray.h"
 #include "raytracer.h"
 #include "renderobject.h"
@@ -12,7 +13,8 @@
 #include "vec3.h"
 
 Colour RayColour(const Ray& ray, const RenderObject& world) {
-    auto hit_record = world.hit(ray, 0.0, kInf);
+    auto hit_record = world.hit(ray, Interval::kPositive);
+    // auto hit_record = world.hit(ray, Interval{0.505, kInf});
 
     if (hit_record.has_value()) {
         // assert(hit_record.front_face);
@@ -21,7 +23,7 @@ Colour RayColour(const Ray& ray, const RenderObject& world) {
 
         if (!hit_record->front_face) {
             // return Colour{0.0, 0.0, 0.0};
-            return Colour{0.5 * (n + Vec3{1, 1, 1})};
+            return Colour::kBlack;
         }
 
         return Colour{0.5 * (n + Vec3{1, 1, 1})};
diff --git a/src/renderobject.h b/src/renderobject.h
index 82e0b45..f57cfb3 100644
--- a/src/renderobject.h
+++ b/src/renderobject.h
@@ -3,6 +3,7 @@
 #include <memory>
 #include <optional>
 
+#include "interval.h"
 #include "ray.h"
 #include "vec3.h"
 
@@ -25,7 +26,7 @@ class RenderObject {
 
     virtual ~RenderObject() = default;
 
-    virtual std::optional<HitRecord> hit(const Ray& ray, double t_min, double t_max) const = 0;
+    virtual std::optional<HitRecord> hit(const Ray& ray, Interval ts) const = 0;
 };
 
 using SharedRenderObject = std::shared_ptr<RenderObject>;
diff --git a/src/renderobjectlist.h b/src/renderobjectlist.h
index 56a3429..8ba93c9 100644
--- a/src/renderobjectlist.h
+++ b/src/renderobjectlist.h
@@ -4,8 +4,8 @@
 #include <optional>
 #include <vector>
 
+#include "interval.h"
 #include "ray.h"
-#include "raytracer.h"
 #include "renderobject.h"
 
 class RenderObjectList : public RenderObject {
@@ -24,15 +24,16 @@ class RenderObjectList : public RenderObject {
 
     void Clear() { objs_.clear(); }
 
-    std::optional<HitRecord> hit(const Ray& ray, double t_min, double t_max) const override {
+    std::optional<HitRecord> hit(const Ray& ray, Interval ts) const override {
         std::optional<HitRecord> closest_hit_record = std::nullopt;
 
-        double closest_t = kInf;
+        double closest_t = ts.max();
 
         for (const auto& obj : objs_) {
-            auto hit_record = obj->hit(ray, t_min, t_max);
+            Interval _ts{ts.min(), closest_t};
+            auto hit_record = obj->hit(ray, _ts);
 
-            if (hit_record.has_value() && hit_record->t < closest_t) {
+            if (hit_record.has_value()) {
                 closest_hit_record = hit_record;
                 closest_t = hit_record->t;
             }
diff --git a/src/sphere.h b/src/sphere.h
index 05f09cc..34b2d81 100644
--- a/src/sphere.h
+++ b/src/sphere.h
@@ -4,6 +4,7 @@
 #include <cmath>
 #include <optional>
 
+#include "interval.h"
 #include "ray.h"
 #include "renderobject.h"
 #include "vec3.h"
@@ -18,7 +19,7 @@ class Sphere : public RenderObject {
     constexpr Point3 centre() const { return centre_; }
     constexpr double radius() const { return radius_; }
 
-    std::optional<HitRecord> hit(const Ray& ray, double t_min, double t_max) const override {
+    std::optional<HitRecord> hit(const Ray& ray, Interval ts) const override {
         HitRecord hit_record;
 
         Vec3 oc = ray.origin() - centre_;
@@ -39,19 +40,19 @@ class Sphere : public RenderObject {
         // smaller of the ts that hit the sphere
         double t = (-b_half - std::sqrt(discr)) / a;
 
-        if (t > t_max) {
+        if (t >= ts.max()) {
             // if the smaller t is already too large, we don't need to check the second one since it
             // will be even larger
 
             return std::nullopt;
         }
 
-        if (t < t_min) {
+        if (t <= ts.min()) {
             // if the smaller t is too small, check the larger one
 
             t = (-b_half + std::sqrt(discr)) / a;
 
-            if (t < t_min || t > t_max) {
+            if (!ts.surronds(t)) {
                 // larger t also out of range: return false
 
                 return std::nullopt;
@@ -64,10 +65,11 @@ class Sphere : public RenderObject {
 
         hit_record.t = t;
         hit_record.p = ray.At(t);
-        hit_record.normal = (hit_record.p - centre_) / radius_;
 
-        if (!hit_record.front_face) {
-            hit_record.normal = -hit_record.normal;
+        if (hit_record.front_face) {
+            hit_record.normal = (hit_record.p - centre_) / radius_;
+        } else {
+            hit_record.normal = -(hit_record.p - centre_) / radius_;
         }
 
         return hit_record;