android - what is ems and why the editfield size is same (width)?

I just started android development, so I have two question in my mind

• First, why the size of edit field does not change its width.




• Secondly, what is the meaning of these two lines

android:layout_weight="1.0"

android:ems="50"

When I used in layout form android:layout_weight= it take value as 0.1, why ?

On field it take 1.0, why ?
what is ems ?

    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

            android:layout_width="fill_parent"
        android:layout_height="wrap_content"

        android:text="@string/login_header" 
         android:gravity="center_horizontal"
         android:textSize="30dp" />

            android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" 
         android:padding="10dp" >


                     android:layout_height="wrap_content"
            android:layout_width="wrap_content"
            android:text="@string/username"
            android:textSize="18dp"/>

                       android:id="@+id/username"
            android:layout_width="5dp"
            android:layout_height="wrap_content"

            android:layout_weight="1.0"
            android:ems="50"
            android:inputType="text"/ >

    

              android:id="@+id/login"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"

        android:text="@string/login_header"
        android:layout_margin="10dp"/>