photoshop : To specify opacity for a layer or group

A layer’s opacity determines to what degree it obscures or reveals the layer beneath it. A layer with 1% opacity appears nearly transparent, whereas one with 100% opacity appears completely opaque.

Note: You cannot change the opacity of a background layer or a locked layer. You can, however, convert a background layer to a regular layer, which does support transparency. See To convert a background into a layer.
Select a layer or group in the Layers palette.
Do one of the following:
In the Layers palette, enter a value in the Opacity text box or drag the Opacity pop-up slider.

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