This is for safety reasons. Take-Off and Landing are the two most critical phases of flight and thus, every additional step to ensure survivability, be it even just adjusting the light, is taken. The interior lights or cabin lights are adjusted to match the exterior environment, so that in case of an emergency, especially if the interior lights fail, your eyes are already accustomed to the light setting and you do not require additional valuable seconds for adjustment to a possibly darker or brighter environment. The lights are usually adjusted at dusk, night or dawn to match the exterior environment. If the cabing lights do not fail during an emergency, the dimmed light also makes it easier to identify the \”EXIT\” signs which illuminate and make the guidance lighting on the floor easier to follow.
The bright emergency lighting is more prominent to identify when the cabin light is dimmed, saving valuable seconds as the aircraft is evacuated. This also ties in with having to raise the blinds on the windows. The blinds need to be raised during take-off and landing irrespective of the outside light situation, however, with a darker cabin, any outside light source, especially from fires or other hazards is easier to identify by the crew, making the decision process of whether one side could be inappropriate to use for evacuation faster, e. g. fire on port side –> no evacation on that side.
The blinds are therefore raised to allow a better view for cabin crew of the outside area and external aircraft parts, such as engines, wings, etc. See also this related question:
Why, when planes are taking off and landing at night, do the lights have to be dimmed? I remember reading somewhere that the reason the interior lights of an aircraft are dimmed/switched-off on landing and taking off is that these are the times the plane is most likely to crash. In the event of a crash, survivors will have a better chance of escape with their eyes adjusted to the dark already. I was told by cabin crew on a BA flight that it was in case of an accident or incident that would require evacuation of the aircraft.
With the likelihood that the interior lighting would fail the passengers eyes would already be some way to being accustomed to the dark. The lights are dimmed in order to reduce electrical load. At takeoff, you want all the power possible in order to shorten the takeoff roll and the more electrical load, the more engine power is sapped off to generate electricity. The same is true on landing, though it is power is reserve in case the flightcrew need to abort the landing in a hurry. They would not want to be messing about with electrical switches at such a busy time. The \”dimming of cabin lights\” only happens when it is dusk, dawn or dark outside the aircraft.
This is a safety measure, and is to ensure your eyes are adjusted to the gloom enough to see the floor lights leading you to safety along the aisle in the event of a crash or emergency evacuation. If the cabin lights were on, but then went out your eyes would need a while to adjust to the lower light levels. I tend to agree with the reason that they are dimmed so that in the event of a crash, your eyes don\’t need any extra time to get adjusted to the darkness. For the same reason, during a day flight, it is insisted that all the windows are kept open so the brightness of the cabin matches the brightness of the day outside.