Lights do not dim when a person walks into or leaves a room, assuming there are no automated systems or sensors that control the lighting. In the absence of such systems, the lighting conditions in a room remain constant regardless of whether someone is present or not.
When a person walks into a room, the lighting level may subjectively appear to change due to the adjustment of our eyes to the new environment. Initially, if you are coming from a bright area, the room might appear dimmer until your eyes adapt to the new lighting conditions. This adaptation process can take a few moments.
Conversely, when you leave a room and step into a brighter environment, the room you just left might momentarily appear darker as your eyes adjust to the increased light. This is known as the adaptation time of the human visual system.
However, it's important to note that these subjective experiences of perceiving changes in light levels are due to the characteristics of our eyes and visual perception, rather than any actual change in the intensity of the lights themselves. The lights in the room typically remain at a constant level unless adjusted manually or controlled by automated systems.