The most serious error designers make with shadows is thinking of it as an effect, and not representing something real. They trick our eye into believing that the object we are seeing is actually above the surface of the page or screen. When you see a page that seems to be alive with depth and dimension, you will, without exception, be looking at expert shadow work. The designer understands what a shadow is all about. And a shadow is about light.
Light falls on a three dimensional object and creates a shadow.
But unless you have a 300-watt, bare light bulb pointed at an object, you are not likely to have a create a shadow that resembles the default setting in most graphics programs. Let's take a look at the blue balls at the left. The final image has been created with the default shadow. It looked more like a ball when it had no shadow. In fact, this shadow has flattened it to looking like a button.