gazebo5 specular in material does not work !

edit

this is my sdf file.

and I expect my ball would reflect blue specular light

but nothing happened,It just appears whole dark

what am I doing wrong??

thanks!

<model name='socket'>
    <pose> 0 0 0 0 0 0 </pose>
    <link name="link">
        <collision name="collision">
            <geometry>
                <sphere>
                    <radius>0.1</radius>
                </sphere>
            </geometry>
        </collision>
        <visual name="visual">
            <material>
                <ambient>0 0 0 1</ambient>
                <diffuse>0 0 0 1</diffuse>
                <specular>0 0 1 1</specular>
            </material>
            <geometry>
                <sphere>
                    <radius>0.1</radius>
                </sphere>
            </geometry>
        </visual>
    </link>
</model>