Setting up…

The card star: it never changes sizeRed clip: your guess for the top of the shadowRail, lamp, screenThe lamp's bulb is 1.0 cm across and 50 cm from the screen. Light travels from it in straight lines, so the star blocks a cone of light, and the cone widens all the way to the screen: the shadow is the star's height times the screen distance over the star's distance from the lamp. At 15 cm from the lamp the 5.4 cm star throws a 18.1 cm shadow; at 44 cm, only 6.2 cm. Because the lamp is not a single point, the shadow's edge is soft, and it gets softer the nearer the star is to the lamp.