You will recall the feeling of darkness when you walk into a movie theater. Or the glare that you experience when you walk out of one. Don’t you feel paralyzed for a fraction of time before you are able to see well. You may notice that when you first walk into a movie theater you slow down and watch your steps to make sure you don’t fall.
The reason is that our eyes have not adapted to the dark or light conditions when we first walk into these lighting conditions. This is nothing but dark or light adaptation.
Here is what happens. When you are standing outside a movie theater in bright light, your pupils are small. Pupils are like the shutter of a camera. They control the amount of light entering the eye. By becoming small they reduce the amount of light that enters the eye. We thus are quite comfortable in bright sunshine. Now, when we walk into a movie theater the pupils are still small. But now the light available outside is very less since the movie theater is dark. Thus the amount of light entering the eye is also very less. We thus are not able to see well in the begining. Yet, in a fraction, the pupils dilate and more light enters the eye and our vision improves even in the dark. The reverse would happen when we step out of the movie theater.
Coming back to the flights. During a flight the time when most accidents happen is during take off and landing. God forbid, if something were to happen, we as passengers and crew have to be alert and be able to move fast. By dimming the cabin lights and opening up the window shades, we are basically creating the same lighting conditions that exist outside. Dark or light. We are thus adapted to the outside lighting conditions. In short, we are being prepared to deal with some mishap.