Since we are diurnal animals (we are awake during the day and sleep at night), during the day we need to be able to see many things about an object: its colours, textures, and contours. At night, though, our eyes don’t need to function as well. In order to see in these two different ways, the eye uses structures called light receptors. These receptors are located on a very thin sheet of tissue, called the retina, located at the back of our eyeball. Light enters the eye through the lens and hits the receptors on the retina. The retina then sends the light via the optic nerve to the brain, where the information is processed.
There are two types of receptors, performing two different functions. The first type, cones, are sensitive to light and colour, and are used mostly for day vision. The other type are rods, which are sensitive to dim light but not to colour. During the day the eyes mostly use cones, so in daylight we see colours. At night, though, the eye uses mostly rods. For this reason we don’t see colours at night very well.
The use of rods and cones has a couple of interesting consequences. At the centre of the retina is a spot called the fovea, which contains only cones. When you focus on an object, that object lands on the fovea, while everything around the object lands on the rest of the retina. Since the fovea contains cones, which are useful for seeing in the daytime, any object that lands on the fovea at night will not be seen. So, if you stare directly at a star in the sky, the image will land on the fovea and you won’t be able to see it very well. If you look just beside the star, however, the star’s image won’t land directly on the fovea. It lands instead in a region around the fovea, which contains rods (remember that rods work well in dim light). You will thus be able to see the star better. Try this when you get home tonight!
Can you think of any animals which need to see well at night? What are the special features of their eyes that help them to do this?