So the reason they dim the cabin lights and insist the shades are raised is (apparently) because ”“Dimming the lights allows your eyes to pre-adjust to darkness, so that you’re not suddenly blinded if something happens and the power goes out, and you’re dashing for the doors in darkness or smoke,” explains Patrick Smith, an airline pilot and author of Cockpit Confidential.
“The emergency path-lighting and signs will also be more visible. And it makes it easier to see outside, which helps you to maintain at least a basic sense of orientation – i.e. which way is up.”
Raising your window shade during take-off and landing is also part of the same strategy.
“It helps you remain oriented, and also makes it easier for the flight attendants to assess any exterior hazards, such as fire or debris, that might interfere with an evacuation,” says Smith.
The human eye can take up to 10 minutes to fully adapt to darkness, which is valuable time if you need to evacuate an aircraft in a hurry.” (http://www.traveller.com.au/why-flight-attendants-dim-the-lights-for-takeoff-and-landing-gtga32). So now I know and so do you.