In this example, tho', nothing happens in the horizontal direction. We can use Newton's Laws to determin the value of the normal force. Tension T acts upward on the lamp while the force of gravity pulls down with force w , the weight of the lamp. The net force is the vector sum of these two forces. The lamp is not accelerating so the force up must equal the force down .
Tension is the magnitude of the force exerted by a chain or a rope or a string.
The direction of that force depends upon the rest of the situation and the object that we are concentrating on at the moment. If we concentrate on the chain shown below, the downward force T' is the force exerted on the chain by the lamp while the upward force T'' is the force exerted on the chain by the ceiling. There is no substitute for good free-body diagrams. Tension T 3 is easy so we will look at the first. In the diagram below, we have drawn in the forces acting on the traffic light.