You might have traveled by air multiple times and each time during takeoff, the lights of the airplanes would have been turned dim. This has confused the passengers since long and many people come up with different theories, as to why the lights of the airplane are turned dim, every time during takeoff.
Confusion prevails as to why lights of airplane are dimmed during takeoff and landing. Some perceive it as a measure to encourage them to keep their books and phone aside since the passengers might get hurt in case of any eventuality. Some feel it is kind of a dramatic finale to make them feel comfortable in the airplane.
However, reality has nothing to do with this and as evident, the real reason for the lights of any airplane being turned dim during takeoff and landing are very different. The real purpose of this exercise is to ensure that the environment in the cabin matches that in the exterior.
This exercise can also be seen as a precautionary measure, in case any evacuation has to be done, either in early morning hours or in late evening hours. If the passengers have to be evacuated through an emergency slide, a person who comes from a brightly lit cabin will find the outside dark ambiance difficult to adjust. Human eyes also need ample time to adjust to normal lighting and if this dimming of lights is done as a regular exercise during takeoff and landings, people will adjust to emergency situations in a better manner.
This is the precise reason for which the passengers are requested to leave the blinds up during descent. Natural light from outside is likely to help keep the view inside normal and in case of any eventuality, evacuation will become easier in natural light conditions.