To create interesting effects, you can vary the opacity of a layer from the default of 100 percent (fully opaque) down to 0 percent (fully transparent). When a layer is partially transparent, the underlying layers show through.
You can also change the opacity of a layer group.
The Opacity setting on the Layers palette determines the opacity of each layer. The overall opacity of a layer and the opacity of individual pixels are independent of each other. For example, if a pixel starts at 50 percent opacity and the layer is set to 50 percent opacity, the pixel appears as 25 percent opaque. If the layer is in a layer group that is set to 50 percent opacity, then the pixel appears as 12.5 percent opaque.
To set the opacity of a layer
On the Edit tab:
1 - On the Layers palette, select the layer or layer group.
2 - Drag the Opacity slider to the desired percentage.
You can also set layer opacity by double-clicking the layer, changing the Opacity setting in the Layer Properties dialog box, and clicking OK.