Because light is a wave, it has the capability to "bend around corners". This is called diffraction, and is illustrated in the adjacent image.

The amount of light reaching a position on the far right is indicated by the orange area under the curve. The intensity of light behind the barrier drops rapidly but is not zero in the shadow region. Diffractive effects occur generally when a part of a light wave is cut off by an obstruction.

Here is a Java applet illustrating diffraction of light by a single slit, and here is an interactive demonstration of refraction and diffraction for ocean waves.