In this short course we will analyze how light and shadow work in the real physical world. Then we’ll look at how we can take those concepts and use them in Adobe Photoshop to produce better shadows and more realistic lighting. Whether you're a graphic designer, concept artist or digital painter, this course will help you improve your digital artwork!
http://design.tutsplus.com/courses/mastering-light-shadow-in-adobe-photoshop
http://design.tutsplus.com/courses/mastering-light-shadow-in-adobe-photoshop