[android-developers] setting the opacity of an activity

i have an activity and would like to set the alpha of the background.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#200000ff"
    android:orientation="vertical" >

i'm setting the above - but i'm not seeing the underlying activity (the one
that started this one).

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