While traveling in a commercial jet, you have likely noticed that the cabin crew dims the light when the plane is about to take off/land. The first time I observed this for myself, I was intrigued, but then I assumed that it must be done as a signal for people to stop whatever activity they’re engaged in (e.g., reading a book, working on a phone/laptop etc.) and start paying attention to the safety instructions demonstrated by the cabin crew.
Just like me, many people still believe that’s why they dim the lights during take off/landing. However, that’s not the real reason behind this distinctive activity.
Why do they dim airplane cabin lights during take off/landing?
Short answer: It’s a safety precaution to ensure that passengers’ eyes adequately adjust to the limited brightness of the cabin, in case something goes wrong during take off or landing, since these are the two most dangerous parts of any flight. It also helps the crew observe the external parts (e.g., engines, wings) to detect any sort of visually observable malfunction.
The role of pupils to regulate the intensity of light
The human eye consists of a tiny component – a ‘hole’ located at the center of the iris – that controls the amount of light entering the eye and striking the retina at the back. This light is then converted into nerve impulses that are transmitted to the brain via the optic nerve.
The pupils always appear black because the light rays that fall on it are either absorbed after diffuse reflections within the eye and do not exit the pupil, or are directly absorbed by the tissues present in the eye. Different creatures have pupils of different shapes; humans’ pupils are round, cats have vertical pupils and goats have horizontally-oriented pupils.
Pupillary light reflex
For the scope of this article, pupillary light reflex is what we are (mostly) interested in. Pupillary light reflex is the eye’s reflex in response to the intensity of light falling on it. In simple words, this is what the eye does to control the luminance of ambient light, so that we can see things regardless of the lightness/darkness around us.
You have surely experienced the discomforting sensation in your eyes when you walk out of a dark theater into broad daylight. That is a perfect example of when the pupillary light reflex is at play. It has the ability to change the diameter of the pupil from as little as 1/16th of an inch to around 1/3 of an inch! (Source)
How is the pupillary light reflex related to the dimming of plane cabin lights?
From the perspective of passenger safety, take off and landing are the two most critical parts of any flight. Historic data shows that most plane accidents occur almost exclusively during these phases.
Therefore, if anything goes wrong during take off/landing that results in a power outage or any other emergency that could potentially limit (or even completely eliminate) the ambient artificial lighting inside the cabin, people should be able to maintain visual awareness and respond to the situation. In a bid to ensure that people’s eyes (more specifically, their eyes’ pupils) are adjusted to the minimal lighting inside and the natural light outside the plane, the cabin lights are dimmed during take off/landing.
Furthermore, limited lighting inside the cabin allows the crew to visually examine and detect any signs of a malfunction on the external components of the aircraft, such as the wings and engines, and quickly take appropriate actions if they sense any threat. That’s also why they ask passengers to pull up the blinds of their respective windows. In rare circumstances (while performing a high-performance take off/landing), pilots turn off all auxiliary systems (that involve cabin lights) to ensure the availability of maximum power in the cockpit for the maneuver.
The dimming of lights, therefore, is a safety measure for passengers to help them evacuate quickly if anything goes wrong in the initial/final moments of a flight.
- Pupillary Light Reflex – Wikipedia
- Pupillary Responses – Stanford Medicine 25
- Pupil – Exploratorium
- The Independent