Hold a straight edge, such as a ruler or a sheet of paper, about 0.5 m to 1.0 m from the bulb of a desk lamp,?

hold a straight edge, such as a ruler or a sheet of paper, about 0.5 m to 1.0 m from the bulb of a desk lamp, and look cloesly at hte edge of the shadow cast on a surface held about 10 centimeters from the straight edge. Can you explain why the shadow is not perfectly sharp?