Question

A man and his son returned home after playing outside for an hour under strong sunlight. The boy complained to his father that their room is unusually dark.

Explain the boy’s observation using your knowledge of how photoreceptors work.   

Answer 1

When we are outside, there is too much light. To compensate for this, our pupils (the part of your eye that light goes through) closes some. And then, when you go back inside our pupils are still small. Inside (even with the lights on) is not nearly as bright as outside. Not enough light is getting through your pupils and so everything looks dark.

One factor is that the eye adjusts its response to light by utilising a form of "gain control" in the nerves in the retina. The signal being sent to the brain is adjusted to allow for the overall brightness. When we come inside into a darker area, the signal is "turned up", so that our vision becomes more sensitive. This doesn't happen instantly, so things appear dark to begin with, but quickly start to appear brighter.

Another mechanism for adjusting the sensitivity of the eye is the regeneration of photopigments in the photoreceptors (the light-sensitive cells in the retina). Photopigments are the molecules that change shape when they absorb light, setting off a chemical reaction that ultimately results in a nerve cell sending a signal down the optic nerve to the brain. Once a photopigment molecule absorbs a photon, it takes a while (and some fancy biochemistry) for it to change back to its original shape, "resetting" it so that it is ready to react to light again. When we come inside there are fewer photons hitting the retina, so more photopigment molecules can be reset. This is a slower process, so our vision becomes more sensitive over a period of a few minutes.

A fourth mechanism comes into play if we move into a really dark area. Your eyes have two types of photoreceptors, known as cones and rods. The cones work well in brighter conditions and allow for color vision because there are 3 types of cones that respond differently to different wavelengths of light. Rods don't work well in bright light, but are very good at detecting small amounts of light. Because there is only one type of rod they can't distinguish colors, and because of the way they are spread out over the retina they aren't very good at seeing fine detail.

As the light gets dimmer, it reaches the point where the cones can't detect the light anymore. At this point the rods take over, allowing the eye to detect very dim light at the cost of losing some detail and color vision. We can observe this if we go into a dark room (or turn off the light at night). Initially we can't see anything, but if we wait, then we can gradually start to make out shapes. After a few minutes we can see quite well, although not as well as with the lights on, and everything is in shades of grey, not color.