Perhaps you are asking about the eternal light? Synagogues have an eternal light in front of or above the ark, the ornate cabinet holding the scrolls of the Torah used for public reading during worship services. The light is a symbol of God's presence and a symbol of the eternal light that used to burn in front of the Holy of Holies in the Temple in Jerusalem back before the Roman Empire destroyed that and made off with the lampstand (you can see the image of that booty on the Arch of Titus in Rome).
The ner tamid, or eternal light, is a reminder of the lamp in the Beis Hamikdash that never went out.
There is no standard item positioned behind the eternal light in a synagogue. It depends on the decor of the synagogue, and not on Jewish law.
need and purpose of internal mobility
The "eternal light" in the synagogue is not a permanent yahrtzeit light - it is a sign of the eternal covenant between G-d and the Jews.
the ark, eternal light and other
No, but it is customary where possible, for synagogues to have windows.
The filament is a special metal composition of tungsten that conducts current and heats up and emits light as a result of the current and its internal resistance.
The local board of directors or council of a synagogue interviews the cantor according to their own internal policies.
The chief purpose is prayer. Services are held every day. In addition, many synagogues have Torah-classes, free of charge; and some have the occasional social function. Congregating in the synagogue also serves to strengthen community and interpersonal ties.
It is when somebody does something on purpose.
Transmitted illumination refers to the process of directing light through a translucent object, such as a microscope slide, in order to view its internal structure or features. This technique is commonly used in microscopy to observe the details of specimens that are best visualized with light passing through them.
The purpose of a lightbulb is to produce light.