Why do they dim lights during takeoff and landing on a plane?

Why do the cabin crew dim the lights on an aircraft during takeoff and landing?

And why do passengers have to raise the window shades?

See also: Why do plane windows have tiny holes in them?

See also: Cameraman captures 'insanely low' takeoff in Caribbean

Takeoff and landing are the most critical phases of a flight, and dimming the lights allows passengers' eyes to adjust more quickly during an emergency evacuation.

This means that if there was a sudden crisis, it would be easier to see the emergency lights lining the aisle and exits.

Natural light can also assist with orientation during an emergency.

Dimming the lights and raising the shades are almost never necessary. But in case of a disaster, they can be life-saving precautions.