Imagine a white paper, a shape behind it, and behind the shape a source of light, like a candle. I want to achieve the shadow effect you perceive looking at the white paper. How to do this in photoshop?
If I'm right in assuming what you're looking for is a similar effect of shadow puppetry, then you can achieve this in Photoshop using multiple layers:
I started with a blank colored background
I added a subtle vignette with a radial gradient
I thenj added my silhouette
I added another on top, enlarged it and gave it a gausian blur and reduced the opacity (You can move this one around to change where the light source is)
Finally, I changed the colors of the blured image to better mimic a shadow