diff --git a/src/program.cpp b/src/program.cpp
index 263130f..8db45fb 100644
--- a/src/program.cpp
+++ b/src/program.cpp
@@ -4,9 +4,27 @@
 
 #include <iostream>
 
+double hit_sphere (const point3& center, double radius, const ray& r) {
+    vec3 oc = r.origin() - center;
+    auto a = dot(r.direction(), r.direction());
+    auto b = 2.0 * dot(oc, r.direction());
+    auto c = dot(oc, oc) - radius*radius;
+    auto discriminant = b*b - 4*a*c;
+    if ( discriminant < 0) {
+        return -1.0;
+    }else{
+        return (-b - sqrt(discriminant) ) / (2.0*a);
+    }
+}
+
 color ray_color(const ray& r){
+    auto t = hit_sphere(point3(0,0,-1), 0.5, r);
+    if(t > 0.0){
+        vec3 N = unit_vector(r.at(t) - vec3(0,0,-1));
+        return 0.5*color(N.x()+1, N.y()+1, N.z()+1);
+    }
     vec3 unit_direction = unit_vector(r.direction());
-    auto t = 0.5*(unit_direction.y() + 1.0);
+    t = 0.5*(unit_direction.y() + 1.0);
     return (1.0-t)*color(1.0, 1.0, 1.0) + t*color(0.5,0.7,1.0);
 }