At night, why does it take my eyes several minutes to get used to darkness?

★ wanayoo — archive 1999 http://howstuffworks.com/question53.htm

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Each weekday Marshall Brain answers questions submitted by the readers of How Stuff Works in the Question of the Day section of HSW. You can also view previously answered questions in the HSW Question Archive. Here is today's question!

Question

Why does it take my eyes several minutes to get used to darkness? If I am asleep and wake up to go to the bathroom, I can see fine in the dark. But then, when I turn on the bathroom light, something happens. When I turn off the light it might take 4 or 5 minutes for my eyes to readjust to the dark. What happened?

Answer

One of the most amazing things about human vision is the incredible range it has. We can see in very bright sunlight, and we can also see in nearly total darkness. If you spend much time working with a camera you know how amazing this range is. Film that works well outdoors is nearly useless indoors, and vice versa. The range that our eyes have comes from three different parts of the eye:

The Pupil. The pupil contracts and expands depending on the amount of light, and can physically block the amount of light entering the eye in bright situations.
The rod and code cells in the retina. Our eyes sense light with two different types if cells: rods and cones. Cone cells can perceive color in bright light. Rod cells perceive black and white images and work best in low light.
A chemical called rhodopsin in the rods.

Rhodopsin is the key to night vision - it is the chemical that the rods use to absorb photons and perceive light. When a molecule of rhodopsin absorbs a photon, it splits into a retinal and an opsin molecule. These molecules later recombine naturally back into rhodopsin at a fixed rate, and recombination is fairly slow.

So, when you expose your eyes to bright light, all of the rhodopsin breaks down into retinal and opsin. If you then turn out the lights and try to see in the dark, you can't. The cones need a lot of light so they are useless, and there is no rhodopsin so the rods are useless too. Over the course of several minutes, however, the retinal and opsin recombine back into rhodopsin and you can see again.

One fun fact - the retinal used in the eye is derived from vitamin A. If a person's diet is low on vitamin A, there is no retinal in the rods and therefore there can be no rhodopsin. People who lack vitamin A therefore suffer from night blindness - they cannot see in the dark.

Interesting Links:

Night Vision
All about Night Vision
Eye Information - see in particular the section on visual sensitivity and adapting to darkness
How we see colors

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