Why are airplane lights turned off sometimes?

Most of us have had travelled in an airplane sometime and must have experienced that the airplane lights are turned off during some situations. These situations most probably are the takeoffs and landings but these situations can also be aircraft part failures.

The reason is something known as "Dark Adaptation".

The normal human eye requires about 20-30 minutes to adapt properly to the new surroundings. This time may vary acoording to the intensity of light in the previous situation. So in case of an emergency our eyes have already adapted to the surroundings and we can act immediately.