Shadows occur when an object blocks the light falling on it so the light can’t reach the surface on the other side of the object. Shadows most often appear black because the visible light cannot make its way past the obstruction, if there is no light falling on an object then it will be black as there is no light to reflect. Shadows are very rarely completely black as there are usually reflections off surfaces and other light sources, which make their way into the shadowy area. As to why black is the absence of light, it just is and we have to accept that. We could delve into philosophy and talk about ‘does black look the same to me as it does to you?’ but since there is no way of knowing and experiencing things from other people’s immediate perspective, then we would really just be going around in circles. Coloured shadows can form if there are multiple coloured light sources or if the light goes through certain translucent objects such as stained glass windows, where only some of the light is blocked.
Author: Rik Sargent, Science Museum