Why can’t we see well right after entering a darker room?

You’re standing on your balcony in the noon sun. It’s bright. After a moment, you step back into a darker room, and for a few moments you’re unable to see clearly. This is because your eyes need time to adjust.

The eyes are made of three layers: sclera and cornea, the pupil, and the retina. The pupil is like a camera’s aperture: by changing the size of its opening, it can control how much light reaches the retina.

Light that’s too bright can damage the retina, so in bright conditions, the pupil’s opening shrinks. If the retina is damaged, it can lead to blurry vision and, in severe cases, lead to permanent loss of vision.

Conversely, in a dimmer room, there’s not enough light, so the pupil opens wider and the retina can sense more light.

So when you suddenly enter the darker room after having been outside in the sun, your pupils are still small. They need a few moments to widen, and until they do, not enough light enters the eye and vision is blurred.

There is another reason why things are not immediately visible in the dark. The retina contains rod cells and cone cells. The rod cells sense brightness using a light-sensitive protein called rhodopsin. Bright light breaks down rhodopsin quickly and the rod cells become inactive. When the eyes adjust to the darkness, the protein’s levels are building back up. And once they do the rod cells become active.

S. Adithya Jyothi is interning with The Hindu.