diff --git a/topics/light.py b/topics/light.py
new file mode 100644
index 00000000..67ca3f39
--- /dev/null
+++ b/topics/light.py
@@ -0,0 +1,512 @@
+from helpers import *
+
+from mobject.tex_mobject import TexMobject
+from mobject import Mobject
+from mobject.vectorized_mobject import *
+
+from animation.animation import Animation
+from animation.transform import *
+from animation.simple_animations import *
+from animation.continual_animation import *
+
+from animation.playground import *
+from topics.geometry import *
+from topics.functions import *
+from scene import Scene
+from camera import Camera
+from mobject.svg_mobject import *
+from topics.three_dimensions import *
+
+from scipy.spatial import ConvexHull
+
+import traceback
+
+LIGHT_COLOR = YELLOW
+SWITCH_ON_RUN_TIME = 1.5
+FAST_SWITCH_ON_RUN_TIME = 0.1
+NUM_LEVELS = 30
+NUM_CONES = 7 # in first lighthouse scene
+NUM_VISIBLE_CONES = 5 # ibidem
+ARC_TIP_LENGTH = 0.2
+AMBIENT_FULL = 0.5
+AMBIENT_DIMMED = 0.2
+SPOTLIGHT_FULL = 0.9
+SPOTLIGHT_DIMMED = 0.2
+
+LIGHT_COLOR = YELLOW
+DEGREES = TAU/360
+
+inverse_power_law = lambda maxint,scale,cutoff,exponent: \
+    (lambda r: maxint * (cutoff/(r/scale+cutoff))**exponent)
+inverse_quadratic = lambda maxint,scale,cutoff: inverse_power_law(maxint,scale,cutoff,2)
+
+
+
+class LightSource(VMobject):
+
+    # combines:
+    # a lighthouse
+    # an ambient light
+    # a spotlight
+    # and a shadow
+
+    CONFIG = {
+        "source_point": ORIGIN,
+        "color": LIGHT_COLOR,
+        "num_levels": 10,
+        "radius": 5,
+        "screen": None,
+        "opacity_function": inverse_quadratic(1,2,1),
+        "max_opacity_ambient": AMBIENT_FULL,
+        "max_opacity_spotlight": SPOTLIGHT_FULL
+    }
+
+    def generate_points(self):
+        print "LightSource.generate_points"
+        self.lighthouse = Lighthouse()
+        self.ambient_light = AmbientLight(
+            source_point = self.source_point,
+            color = self.color,
+            num_levels = self.num_levels,
+            radius = self.radius,
+            opacity_function = self.opacity_function,
+            max_opacity = self.max_opacity_ambient
+        )
+        if self.has_screen():
+            self.spotlight = Spotlight(
+                source_point = self.source_point,
+                color = self.color,
+                num_levels = self.num_levels,
+                radius = self.radius,
+                screen = self.screen,
+                opacity_function = self.opacity_function,
+                max_opacity = self.max_opacity_spotlight
+            )
+        else:
+            self.spotlight = Spotlight()
+
+        self.shadow = VMobject(fill_color = "BLACK", fill_opacity = 1.0, stroke_color = BLACK)
+        self.lighthouse.next_to(self.source_point,DOWN,buff = 0)
+        self.ambient_light.move_source_to(self.source_point)
+
+        if self.has_screen():
+            self.spotlight.move_source_to(self.source_point)
+            self.update_shadow()
+
+        self.add(self.ambient_light,self.spotlight,self.lighthouse, self.shadow)
+
+    def has_screen(self):
+        return (self.screen != None)
+
+    def dim_ambient(self):
+        self.set_max_opacity_ambient(AMBIENT_DIMMED)
+
+    def set_max_opacity_ambient(self,new_opacity):
+        self.max_opacity_ambient = new_opacity
+        self.ambient_light.dimming(new_opacity)
+
+    def dim_spotlight(self):
+        self.set_max_opacity_spotlight(SPOTLIGHT_DIMMED)
+
+    def set_max_opacity_spotlight(self,new_opacity):
+        self.max_opacity_spotlight = new_opacity
+        self.spotlight.dimming(new_opacity)
+
+    def set_screen(self, new_screen):
+        if self.has_screen():
+            self.spotlight.screen = new_screen
+        else:
+            self.remove(self.spotlight)
+            self.spotlight = Spotlight(
+                source_point = self.source_point,
+                color = self.color,
+                num_levels = self.num_levels,
+                radius = self.radius,
+                screen = new_screen
+            )
+            self.spotlight.move_source_to(self.source_point)
+            self.add(self.spotlight)
+        
+        # in any case
+        self.screen = new_screen
+
+
+
+
+    def move_source_to(self,point):
+        print "LightSource.move_source_to"
+        apoint = np.array(point)
+        v = apoint - self.source_point
+        self.source_point = apoint
+        self.lighthouse.next_to(apoint,DOWN,buff = 0)
+        self.ambient_light.move_source_to(apoint)
+        #if self.has_screen():
+        #    self.spotlight.move_source_to(apoint)
+        #self.update()
+        return self
+
+
+
+
+
+
+        
+    def set_radius(self,new_radius):
+        self.radius = new_radius
+        self.ambient_light.radius = new_radius
+        self.spotlight.radius = new_radius
+
+    def update(self):
+        print "LightSource.update"
+        self.spotlight.update_sectors()
+        self.update_shadow()
+
+
+    def update_shadow(self):
+        
+        point = self.source_point
+        projected_screen_points = []
+        if not self.has_screen():
+            return
+        for point in self.screen.get_anchors():
+            projected_screen_points.append(self.spotlight.project(point))
+
+        projected_source = project_along_vector(self.source_point,self.spotlight.projection_direction())
+
+        projected_point_cloud_3d = np.append(projected_screen_points,
+            np.reshape(projected_source,(1,3)),axis = 0)
+        rotation_matrix = z_to_vector(self.spotlight.projection_direction())
+        back_rotation_matrix = rotation_matrix.T # i. e. its inverse
+
+        rotated_point_cloud_3d = np.dot(projected_point_cloud_3d,back_rotation_matrix.T)
+        # these points now should all have z = 0
+        point_cloud_2d = rotated_point_cloud_3d[:,:2]
+        # now we can compute the convex hull
+        hull_2d = ConvexHull(point_cloud_2d) # guaranteed to run ccw
+        hull = []
+
+        # we also need the projected source point
+        source_point_2d = np.dot(self.spotlight.project(self.source_point),back_rotation_matrix.T)[:2]
+        
+        index = 0
+        for point in point_cloud_2d[hull_2d.vertices]:
+            if np.all(point - source_point_2d < 1.0e-6):
+                source_index = index
+                continue
+            point_3d = np.array([point[0], point[1], 0])
+            hull.append(point_3d)
+            index += 1
+
+        index = source_index
+        
+        hull_mobject = VMobject()
+        hull_mobject.set_points_as_corners(hull)
+        hull_mobject.apply_matrix(rotation_matrix)
+
+
+        anchors = hull_mobject.get_anchors()
+
+        # add two control points for the outer cone
+
+
+        ray1 = anchors[index - 1] - projected_source
+        ray1 = ray1/np.linalg.norm(ray1) * 100
+        ray2 = anchors[index] - projected_source
+        ray2 = ray2/np.linalg.norm(ray2) * 100
+        outpoint1 = anchors[index - 1] + ray1
+        outpoint2 = anchors[index] + ray2
+
+        new_anchors = anchors[:index]
+        new_anchors = np.append(new_anchors,np.array([outpoint1, outpoint2]),axis = 0)
+        new_anchors = np.append(new_anchors,anchors[index:],axis = 0)
+        self.shadow.set_points_as_corners(new_anchors)
+
+        # shift it one unit closer to the camera so it is in front of the spotlight
+        #self.shadow.shift(-500*self.projection_direction())
+        self.shadow.mark_paths_closed = True
+
+
+
+class SwitchOn(LaggedStart):
+    CONFIG = {
+        "lag_ratio": 0.2,
+        "run_time": SWITCH_ON_RUN_TIME
+    }
+
+    def __init__(self, light, **kwargs):
+        if (not isinstance(light,AmbientLight) and not isinstance(light,Spotlight)):
+            raise Exception("Only AmbientLights and Spotlights can be switched on")
+        LaggedStart.__init__(self,
+            FadeIn, light, **kwargs)
+
+
+class SwitchOff(LaggedStart):
+    CONFIG = {
+        "lag_ratio": 0.2,
+        "run_time": SWITCH_ON_RUN_TIME
+    }
+
+    def __init__(self, light, **kwargs):
+        if (not isinstance(light,AmbientLight) and not isinstance(light,Spotlight)):
+            raise Exception("Only AmbientLights and Spotlights can be switched off")
+        light.submobjects = light.submobjects[::-1]
+        LaggedStart.__init__(self,
+            FadeOut, light, **kwargs)
+        light.submobjects = light.submobjects[::-1]
+
+
+
+
+class Lighthouse(SVGMobject):
+    CONFIG = {
+        "file_name" : "lighthouse",
+        "height" : 0.5
+    }
+
+    def move_to(self,point):
+        self.next_to(point, DOWN, buff = 0)
+
+
+class AmbientLight(VMobject):
+
+    # Parameters are:
+    # * a source point
+    # * an opacity function
+    # * a light color
+    # * a max opacity
+    # * a radius (larger than the opacity's dropoff length)
+    # * the number of subdivisions (levels, annuli)
+
+    CONFIG = {
+        "source_point" : ORIGIN,
+        "opacity_function" : lambda r : 1.0/(r+1.0)**2,
+        "color" : LIGHT_COLOR,
+        "max_opacity" : 1.0,
+        "num_levels" : 10,
+        "radius" : 5.0
+    }
+
+    def generate_points(self):
+        print "AmbientLight.generate_points"
+        self.source_point = np.array(self.source_point)
+
+        # in theory, this method is only called once, right?
+        # so removing submobs shd not be necessary
+        for submob in self.submobjects:
+            self.remove(submob)
+
+        # create annuli
+        self.radius = float(self.radius)
+        dr = self.radius / self.num_levels
+        for r in np.arange(0, self.radius, dr):
+            alpha = self.max_opacity * self.opacity_function(r)
+            annulus = Annulus(
+                inner_radius = r,
+                outer_radius = r + dr,
+                color = self.color,
+                fill_opacity = alpha
+            )
+            annulus.move_arc_center_to(self.source_point)
+            self.add(annulus)
+
+
+
+    def move_source_to(self,point):
+
+
+        for line in traceback.format_stack():
+            print line.strip()
+
+        print "AmbientLight.move_source_to blablub"
+        v = np.array(point) - self.source_point
+        print "test"
+        self.source_point = np.array(point)
+        self.shift(v)
+        return self
+
+
+
+
+
+
+
+    def dimming(self,new_alpha):
+        old_alpha = self.max_opacity
+        self.max_opacity = new_alpha
+        for submob in self.submobjects:
+            old_submob_alpha = submob.fill_opacity
+            new_submob_alpha = old_submob_alpha * new_alpha / old_alpha
+            submob.set_fill(opacity = new_submob_alpha)
+
+
+class Spotlight(VMobject):
+
+    CONFIG = {
+        "source_point" : ORIGIN,
+        "opacity_function" : lambda r : 1.0/(r/2+1.0)**2,
+        "color" : LIGHT_COLOR,
+        "max_opacity" : 1.0,
+        "num_levels" : 10,
+        "radius" : 5.0,
+        "screen" : None,
+        "camera": None
+    }
+
+    def projection_direction(self):
+        if self.camera == None:
+            return OUT
+        else:
+            v = self.camera.get_cartesian_coords()
+            return v/np.linalg.norm(v)
+
+    def project(self,point):
+        v = self.projection_direction()
+        w = project_along_vector(point,v)
+        return w
+
+    def generate_points(self):
+
+        self.submobjects = []
+
+        if self.screen != None:
+            # look for the screen and create annular sectors
+            lower_angle, upper_angle = self.viewing_angles(self.screen)
+            self.radius = float(self.radius)
+            dr = self.radius / self.num_levels
+            lower_ray, upper_ray = self.viewing_rays(self.screen)
+
+            for r in np.arange(0, self.radius, dr):
+                new_sector = self.new_sector(r,dr,lower_angle,upper_angle)
+                self.add(new_sector)
+
+        #self.update_shadow(point = self.source_point)
+        #self.add_to_back(self.shadow)
+
+    def new_sector(self,r,dr,lower_angle,upper_angle):
+
+        alpha = self.max_opacity * self.opacity_function(r)
+        annular_sector = AnnularSector(
+            inner_radius = r,
+            outer_radius = r + dr,
+            color = self.color,
+            fill_opacity = alpha,
+            start_angle = lower_angle,
+            angle = upper_angle - lower_angle
+        )
+        # rotate (not project) it into the viewing plane
+        rotation_matrix = z_to_vector(self.projection_direction())
+        annular_sector.apply_matrix(rotation_matrix)
+        # now rotate it inside that plane
+        rotated_RIGHT = np.dot(RIGHT, rotation_matrix.T)
+        projected_RIGHT = self.project(RIGHT)
+        omega = angle_between_vectors(rotated_RIGHT,projected_RIGHT)
+        annular_sector.rotate(omega, axis = self.projection_direction())
+        annular_sector.move_arc_center_to(self.source_point)
+
+        return annular_sector
+
+    def viewing_angle_of_point(self,point):
+        # as measured from the positive x-axis
+        v1 = self.project(RIGHT)
+        v2 = self.project(np.array(point) - self.source_point)
+        absolute_angle = angle_between_vectors(v1, v2)
+        # determine the angle's sign depending on their plane's
+        # choice of orientation. That choice is set by the camera
+        # position, i. e. projection direction
+        if np.dot(self.projection_direction(),np.cross(v1, v2)) > 0:
+            return absolute_angle
+        else:
+            return -absolute_angle
+
+
+    def viewing_angles(self,screen):
+
+        screen_points = screen.get_anchors()
+        projected_screen_points = map(self.project,screen_points)
+
+        viewing_angles = np.array(map(self.viewing_angle_of_point,
+            projected_screen_points))
+        lower_angle = upper_angle = 0
+        if len(viewing_angles) != 0:
+            lower_angle = np.min(viewing_angles)
+            upper_angle = np.max(viewing_angles)
+
+        return lower_angle, upper_angle
+
+    def viewing_rays(self,screen):
+
+        lower_angle, upper_angle = self.viewing_angles(screen)
+        projected_RIGHT = self.project(RIGHT)/np.linalg.norm(self.project(RIGHT))
+        lower_ray = rotate_vector(projected_RIGHT,lower_angle, axis = self.projection_direction())
+        upper_ray = rotate_vector(projected_RIGHT,upper_angle, axis = self.projection_direction())
+        
+        return lower_ray, upper_ray
+
+
+    def opening_angle(self):
+        l,u = self.viewing_angles(self.screen)
+        return u - l
+
+    def start_angle(self):
+        l,u = self.viewing_angles(self.screen)
+        return l
+
+    def stop_angle(self):
+        l,u = self.viewing_angles(self.screen)
+        return u
+
+    def move_source_to(self,point):
+        self.source_point = np.array(point)
+        self.update_sectors()
+        return self
+
+
+    def update_sectors(self):
+        if self.screen == None:
+            return
+        for submob in self.submobject_family():
+            if type(submob) == AnnularSector:
+                lower_angle, upper_angle = self.viewing_angles(self.screen)
+                dr = submob.outer_radius - submob.inner_radius
+                new_submob = self.new_sector(submob.inner_radius,dr,lower_angle,upper_angle)
+                submob.points = new_submob.points
+
+
+
+
+
+
+    def dimming(self,new_alpha):
+        old_alpha = self.max_opacity
+        self.max_opacity = new_alpha
+        for submob in self.submobjects:
+            if type(submob) != AnnularSector:
+                # it's the shadow, don't dim it
+                continue
+            old_submob_alpha = submob.fill_opacity
+            new_submob_alpha = old_submob_alpha * new_alpha/old_alpha
+            submob.set_fill(opacity = new_submob_alpha)
+
+    def change_opacity_function(self,new_f):
+        self.opacity_function = new_f
+        dr = self.radius/self.num_levels
+
+        sectors = []
+        for submob in self.submobjects:
+            if type(submob) == AnnularSector:
+                sectors.append(submob)
+
+        for (r,submob) in zip(np.arange(0,self.radius,dr),sectors):
+            if type(submob) != AnnularSector:
+                # it's the shadow, don't dim it
+                continue
+            alpha = self.opacity_function(r)
+            submob.set_fill(opacity = alpha)
+
+
+
+class ScreenTracker(ContinualAnimation):
+
+    def update_mobject(self, dt):
+        self.mobject.update()
+